VBA to check cell value for NULL then replace with another value

Hi Experts,

I have a macro that someone else wrote but I have to make adjustments to.  I need some help to figure out the best way to acheive the following:

If Last Name = null look in the first name column.  Find the space in the first name column then cut the text after the space then past to the last name column.  

Below is current code.  When it runs and the last name = Null the merged field is empty.

'Name************************************************************
'first
Cells(1, 1).Select
k = 1

Do While First_name <> "RAFirst"
If Cells(1, k).Value <> "RAFirst" Then
k = k + 1
Else
First_name = "RAFirst"
End If

Loop
First_name = k



Columns(First_name).Select
Selection.Copy
Columns("N:N").Select
Selection.Insert Shift:=xlToRight

'last
Cells(1, 1).Select
k = 1

Do While Last_name <> "RALast"
If Cells(1, k).Value <> "RALast" Then
k = k + 1
Else
Last_name = "RALast"
End If

Loop
Last_name = k



Columns(Last_name).Select
Selection.Copy
Columns("O:O").Select
Selection.Insert Shift:=xlToRight

'merge 1st and LastNames
Columns("N:N").Select
Selection.Insert Shift:=xlToRight

For i = 2 To j

Cells(i, 14) = Cells(i, 15) & " " & Cells(i, 16)
Next i

Columns("N:N").Select
Selection.Copy

Cells(1, 14).Select
Selection.PasteSpecial Paste:=xlPasteValues, Operation:=xlNone, SkipBlanks _
        :=False, Transpose:=False