How do I replace blank spaces with a value of 0 or null?

I am writing a program to import information from an Excel spreadsheet and insert the data into a SQL Server database table.  The problem I am faced with here is that the column CAT has blank spaces in the Excel sheet.  How do I replace the blank space value with a 0 or null in VB.NET?  The first record that I tried worked fine because in the column CAT from the Excel spreadsheet, I put in a 0.  But how would I replace the blank values with a 0 or null if I was faced with a spreadsheet that gave me that type of problem using my Insert or doing something with the Insert string in my code?  Below is my code for the program.

Function ReadExcelsheet(ByVal Path As String, Optional ByVal Sheetvar As String = "Sheet1") As System.Data.DataSet

        Dim ds As New DataSet
        Dim da As New OleDbDataAdapter
        Dim conn_string As OleDbConnection
        Dim excel_cmd As New OleDbCommand
        Dim dt As New System.Data.DataTable

        Dim sql_ds As New DataSet
        Dim sql_dbadapter As New SqlDataAdapter
        Dim conn_sql As SqlConnection
        Dim sql_cmd As New SqlCommand
        Dim sql_dt As New System.Data.DataTable
        Dim cmdbuilder As New SqlCommandBuilder
        Dim tester As String





        'Make sure the text box has a string to find an Excel spreadsheet
        If TextBox1.Text = "" Or ComboBox1.Text = "" Then
            MsgBox("No spreadsheets have been loaded yet or no tables have been selected.")
            Exit Function
        Else
            'This is where the connection string is used

            Path = TextBox1.Text
            conn_string = New OleDbConnection( _
                     "Provider= Microsoft.Jet.OLEDB.4.0;" & _
                     "Data Source =" & Path & ";" & _
                     "Extended Properties=""Excel 8.0;HDR=Yes;IMEX=1"";")

            'This is where the connection string reaches the SQL Server database
            conn_sql = New SqlConnection( _
    "Data Source=BRS-5C2ULHTC9ZV;Initial Catalog=NIPWork;Integrated Security=True")

            'The command connecting to a database
            excel_cmd.Connection = conn_string


            'The command string's command text
            excel_cmd.CommandText = "SELECT * FROM [" & Sheetvar & "]"

            sql_cmd.Connection = conn_sql

            'sql_cmd.CommandText = "SELECT * FROM jhirniak.NIP_Valuation"

            ' sql_cmd.CommandText = "SELECT * FROM test_nip_valuation_nonproduction"



            'da = New OleDbDataAdapter("SELECT * FROM [" & Sheet & "$]", conn_string)
            'Since the OleDbCommand stores the connection string and the command text, the line below
            'is the same as the line above.
            da.SelectCommand = excel_cmd

            'Open the connection string
            conn_string.Open()
            If conn_string.State = ConnectionState.Open Then
                MsgBox("The connection string has connected to the source.")
            Else
                MsgBox("Unable to connect")
            End If

            sql_dbadapter.SelectCommand = sql_cmd



            Try
                conn_sql.Open()
            Catch ex As Exception
                MsgBox("Unable to connect to the SQL Server database", MsgBoxStyle.Critical, "Error!")
                Exit Function
            End Try
            'Checks to see if the sql connection is open or not
            If conn_sql.State = ConnectionState.Open Then
                MsgBox("The SQL Connection is now Open")
            Else
                MsgBox("No SQL database connection was made.")
            End If



            'The data adapter fills the dataset with the table information from the database
            da.Fill(ds)
            ' da.Fill(ds, "Test_NIP_Valuation_nonproduction")
            da.Fill(dt)

            'The Sql server data adapter fills the dataset with the table information from the database
            ' sql_dbadapter.Fill(sql_ds)
            'The Sql server dataadapter fills the data table
            ' sql_dbadapter.Fill(sql_dt)





            Dim row As DataRow

            Dim retInt As Integer
            Dim count As Integer = 0
            For Each row In ds.Tables(0).Rows

                If Not row.Item("CAT").ToString = " " Then
                    row.Item("CAT").ToString.Insert(0, 0)
                End If

                tester = "insert into tblNIPIMPORT_Valuation (LineofBusiness, PolicyNO, OccuranceNO, PolOccNo, " & _
                   "PolComm, LossDay, CAT, RiskState, FirstNotice, ValnMo, AccYr, PdIndem, PdALAE, " & _
                  "OSIndem, OSALAE, IncdIndem, IncdALAE, IncdLandALAE, Insured, LossDescription) " & _
                 "values('" & row.Item("LOB").ToString & "', '" & row.Item("Pol #").ToString & "', '" & _
                row.Item("Occ #").ToString & "', '" & row.Item("Pol Occ #").ToString & "', '" & _
                 row.Item("Pol Comm").ToString & "', '" & row.Item("Loss Day").ToString & "', " & _
                row.Item("CAT").ToString & ", " & _
                row.Item("RiskState").ToString & ", '" & row.Item("First Notice").ToString & "', " & _
                row.Item("Valn Mo").ToString & ", " & row.Item("Acc Yr").ToString & ", " & _
                row.Item("PdIndem").ToString & ", " & row.Item("PdALAE").ToString & ", " & _
                row.Item("OS Indem").ToString & ", " & row.Item("OS ALAE").ToString & ", " & _
                row.Item("IncdIndem").ToString & ", " & row.Item("IncdALAE").ToString & ", " & _
                row.Item("Incd L&ALAE").ToString & ", '" & row.Item("Insured").ToString & "', '" & _
                row.Item("Loss Description") & "')"









                sql_cmd = New SqlCommand(tester, conn_sql)

                sql_cmd.ExecuteNonQuery()
                If retInt = 1 Then
                    count = count + 1
                    Continue For
                End If
                'build update
                MsgBox(tester)
                'row.SetAdded()
                count = count + 1
            Next



        End If


        ' sql_dbadapter.Update(sql_ds, tester)




        sql_dbadapter.Update(sql_ds)




        ReadExcelsheet = ds
        DataGridView1.DataSource = dt
        DataGridView2.DataSource = sql_dt
        ' UpdateSQLTable(ds)
        conn_string.Close()
    End Function