I'm not sure what you are trying to do exactly, however if you use the following method you will always write the string to the bookmark regardless if the string is null or not. Doing so always destroys and recreates the bookmark:
Code:
Sub ScratchMacro()
'A basic Word macro coded by Greg Maxey
Dim oRng As Word.Range
Dim strSomething As String
Set oRng = ActiveDocument.Bookmarks("bmBookmarkName").Range
oRng.Text = strSomething
ActiveDocument.Bookmarks.Add "bmBookmarkName", oRng
End Sub