Make artist_name Optional in find_library_artist_for_source

Cin's review note: typing artist_name as plain `str` forced callers
that didn't have a name to pass `""` as a placeholder, which leaks the
parameter's emptiness contract into every call site and reads badly in
tests. Switching to `Optional[str] = None` lets callers omit it.

The function body's `if artist_name and active_server:` check already
handles None and "" identically, so no body changes were needed. Tests
that previously passed `artist_name=""` drop the argument; one new test
covers the omitted-arg path explicitly.

The web_server.py wrapper takes the same default for symmetry.

core/artist_source_lookup.py

@@ -43,7 +43,7 @@ def find_library_artist_for_source(
     database,
     source: str,
     source_artist_id: str,
-    artist_name: str,
+    artist_name: Optional[str] = None,
     active_server: Optional[str] = None,
 ) -> Optional[str]:
     """Return the library PK of an artist matching the source-aware click.

tests/test_artist_source_lookup.py

@@ -109,9 +109,7 @@ class TestFindLibraryArtistForSource:
             **{column: source_value},
         )
 
-        result = find_library_artist_for_source(
-            db, source, source_value, artist_name=""
-        )
+        result = find_library_artist_for_source(db, source, source_value)
         assert result == f"pk-{source}"
 
     def test_unknown_source_returns_none(self, db):
@@ -121,8 +119,16 @@ class TestFindLibraryArtistForSource:
 
     def test_lookup_misses_when_source_id_unknown(self, db):
         _insert_artist(db, artist_id="pk-real", name="Real Artist", deezer_id="dz-real")
+        assert find_library_artist_for_source(db, "deezer", "dz-not-real") is None
+
+    def test_artist_name_is_optional(self, db):
+        """Callers that don't have a name handy should be able to omit it
+        without falling through to the name-fallback branch."""
+        _insert_artist(db, artist_id="pk-q", name="Some Artist", server_source="plex")
+        # No source-id match, no name passed → must return None even when
+        # active_server is set (otherwise we'd risk matching by None name).
         assert find_library_artist_for_source(
-            db, "deezer", "dz-not-real", artist_name=""
+            db, "deezer", "no-id-match", active_server="plex"
         ) is None
 
     def test_name_fallback_matches_within_active_server(self, db):

web_server.py

@@ -11323,7 +11323,7 @@ from core.artist_source_lookup import (
 )
 
 
-def _find_library_artist_for_source(database, source, source_artist_id, artist_name):
+def _find_library_artist_for_source(database, source, source_artist_id, artist_name=None):
     """Thin wrapper that injects the active-server context for the core lookup."""
     try:
         active_server = config_manager.get_active_media_server()