refactor(usage): final round-6 nits on PR #871

- semantic.py: normalize both None and [] doc_types to null in the
  metadata so a future `metadata->'doc_types' IS NULL` query counts the
  all-types case consistently.
- test: use a fixed past date in test_occurred_at_roundtrip instead of a
  future literal (deterministic, no "why this date" confusion).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/server/semantic.py

@@ -548,10 +548,13 @@ def configure_semantic_tools(mcp: FastMCP):
                             "user_id": username,
                             "fusion": fusion,
                             # Bounded copy — see _USAGE_METADATA_MAX_DOC_TYPES.
+                            # Both None and [] normalize to null so a future
+                            # metadata->'doc_types' IS NULL query counts the
+                            # all-types case consistently.
                             "doc_types": (
                                 doc_types[:_USAGE_METADATA_MAX_DOC_TYPES]
                                 if doc_types
-                                else doc_types
+                                else None
                             ),
                         },
                         # The outer guard already confirmed the flag, so pass