Fix db version being accessed before creation if table fails to create

This should prevent the script from hard crashing like in #288

scripts/tag_frequency_db.py

@@ -78,6 +78,7 @@ class TagFrequencyDb:
         )
 
     def __get_version(self):
+        db_version = None
         with transaction() as cursor:
             cursor.execute(
                 """