Bug-fix: need to return None for empty query

Author: schlatterbeck

roundup/backends/rdbms_common.py

@@ -2419,7 +2419,7 @@ def _filter_sql (self, search_matches, filterspec, srt=[], grp=[], retr=0):
                         # where clause will always be false, and we
                         # can optimize the query away.
                         if not v:
-                            return []
+                            return None
                         s = ','.join([a for x in v])
                         where.append('_%s.%s in (%s)'%(pln, k, s))
                         args = args + v