fix(proxy): handle None values in daily spend sort key (#16245)

Fixes TypeError when sorting daily spend transactions with missing entity IDs (e.g., null tags).
Changed x[1].get(entity_id_field) to x[1].get(entity_id_field) or "" to provide a sortable
default value when the field is None, preventing comparison errors between NoneType and str.

litellm/proxy/db/db_spend_update_writer.py

@@ -997,7 +997,7 @@ class DBSpendUpdateWriter:
                             # should sort by the table first.
                             key=lambda x: (
                                 x[1]["date"],
-                                x[1].get(entity_id_field),
+                                x[1].get(entity_id_field) or "",
                                 x[1]["api_key"],
                                 x[1]["model"],
                                 x[1]["custom_llm_provider"],