SQL Where Clause Help

I have the following code and I do not know how to modify it for the following..... I need to add a where clause for the following.

Where Referral_Source is NOT NULL OR BLANK/EMPTY.

SELECT NewBusinessHeader.Exec_Name, NewBusinessHeader.Client_Last_Name, NewBusinessLineItem.EffectiveDate, NewBusinessLineItem.Carrier, NewBusinessLineItem.LineOfBusiness, NewBusinessLineItem.SoldPremium, NewBusinessLineItem.Sold, NewBusinessHeader.Referral_Source,NewBusinessHeader.Branch,NewBusinessHeader.Dept,NewBusinessHeader.Customer_Number as Agency_Rev,NewBusinessHeader.Process_Complete_Date,NewBusinessHeader.Group_Submission_Number

FROM NewBusinessHeader INNER JOIN NewBusinessLineItem ON NewBusinessHeader.NBHeaderID = NewBusinessLineItem.NBHeaderID

WHERE ((NewBusinessLineItem.Sold='Yes')) AND (([NewBusinessHeader].[Process_Complete_Date])>=<<Enter the From Date[Date]>> AND [NewBusinessHeader].[Process_Complete_Date]<= <<Enter the To Date[Date]>>) AND (((NewBusinessHeader.Dept) In (<<Enter Personal Lines, Commercial Lines, Benefit lines (seperated by a comma for mulitple) [Text]+>>)))

UNION ALL

SELECT Submissions.Exec_Name, Submissions.Client_Last_Name,Submissions.EffectiveDate,Submissions.Carrier, Submissions.LineOfBusiness,Submissions.SoldPremium,Submissions.Sold,Submissions.Referral_Source,Submissions.Branch,Submissions.Dept,Submissions.Agency_Rev,Submissions.Process_Complete_Date,Submissions.Group_Submission_Number

FROM Submissions

WHERE ((Submissions.Sold='Yes')) AND Process_Complete_Date >= <<Enter the From Date[Date]>> AND Process_Complete_Date<= <<Enter the To Date[Date]>> AND (((Submissions.Dept) In (<<Enter Personal Lines, Commercial Lines, Benefit lines (seperated by a comma for mulitple) [Text]+>>)))

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