fix: return all notes when search query is empty

Previously, an empty query string to nc_notes_search_notes would return
zero results due to an early return when no query tokens were present.

This was counterintuitive - users expect an empty query to list all
notes, not return nothing.

Changes:
- Modified NotesSearchController.search_notes() to return all notes
  when query is empty
- Added documentation to clarify this behavior
- Empty query results have _score: None (no relevance scoring)
- Non-empty query results continue to have relevance scores

Fixes behavior where listing all notes was impossible via the search tool.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

nextcloud_mcp_server/controllers/notes_search.py

@@ -12,13 +12,24 @@ class NotesSearchController:
         """
         Search notes using token-based matching with relevance ranking.
         Returns notes sorted by relevance score.
+        If query is empty, returns all notes.
         """
         search_results = []
         query_tokens = self._process_query(query)
 
-        # If empty query after processing, return empty results
+        # If empty query after processing, return all notes
         if not query_tokens:
-            return []
+            async for note in notes:
+                search_results.append(
+                    {
+                        "id": note.get("id"),
+                        "title": note.get("title"),
+                        "category": note.get("category"),
+                        "modified": note.get("modified"),
+                        "_score": None,  # No score for unfiltered results
+                    }
+                )
+            return search_results
 
         # Process and score each note
         async for note in notes: