Remove conflation of None and '' query in web UI

Without this conflation, the behavior of handling
empty string query args can be made more consistent
across the app more easily. A proper message will
be generated to the user if they bypass the frontend
to submit an empty query string.

datalad_registry/overview.py

@@ -39,7 +39,7 @@ def overview():  # No type hints due to mypy#7187.
     # as we would add search to individual files.
     query = request.args.get("query", None, type=str)
     search_error = None
-    if query:
+    if query is not None:
         lgr.debug("Search by '%s'", query)
         try:
             criteria = parse_query(query)