GetString (and I suppose Get"AnyType") returns an object, then this is not a bug but the help should be correct.
B4X:
...
Dim Result As String
If Cur.IsInitialized AND Cur.RowCount > 0 Then
Cur.Position = 0
Result = Cur.GetString(FieldName)
if Result = "" Then <--- gets error
If Result = Null Then <--- OK