memory (sqlite): don't store empty messages

sat/memory/sqlite.py

@@ -442,6 +442,9 @@ class SqliteStorage(object):
         d_list = []
         for key in ('message', 'subject'):
             for lang, value in data[key].items():
+                if not value.strip():
+                    # no need to store empty messages
+                    continue
                 d = self.dbpool.runQuery(
                     "INSERT INTO {key}(history_uid, {key}, language) VALUES (?,?,?)"
                     .format(key=key),