Handle memory db indexer test

One of the tests passes a None object to the indexer as part of
unsetting a string. If the text is False, return an empty list.
As "", None don't add info to the indexer.

Author: rouilj

roundup/backends/indexer_dbm.py

@@ -131,6 +131,9 @@ def splitter(self, text, ftype):
     def text_splitter(self, text):
         """Split text/plain string into a list of words
         """
+        if not text:
+            return []
+        
         # case insensitive
         text = text.upper()