IF EXISTS, THEN SELECT ELSE INSERT AND THEN SELECT

How do you say the following in Microsoft SQL Server 2005:

IF EXISTS (SELECT * FROM Table WHERE FieldValue='') THEN
   SELECT TableID FROM Table WHERE FieldValue=''
ELSE
   INSERT INTO TABLE(FieldValue) VALUES('')
   SELECT TableID FROM Table WHERE TableID=SCOPE_IDENTITY()
END IF

What I'm trying to do is to see if there is a blank fieldvalue already, and if there is then return that TableID, else insert a blank fieldvalue and return the corresponding primary key.