fix: dont send an empty series request (#61)

Co-authored-by: Michał Sośnicki <[email protected]>

Author: michalsosn

src/neptune_query/internal/retrieval/series.py

@@ -184,7 +184,7 @@ def _make_new_series_page_params(
         if series.HasField("searchAfter")
     }
 
-    if not updated_request_tokens:
+    if not updated_request_tokens:  # no progress, stop fetching
         return None
 
     new_requests = [
@@ -193,5 +193,8 @@ def _make_new_series_page_params(
         if request["requestId"] in existing_series and request["requestId"] not in finished_requests
     ]
 
+    if not new_requests:  # all requests are finished
+        return None
+
     params["requests"] = new_requests
     return params