fix(subsonic): always include mandatory title field in Child responses

Removed `omitempty` from the `Title` struct tag in the `Child` response
type. The Subsonic/OpenSubsonic API spec requires `title` to be a
mandatory field, but songs with empty titles caused the field to be
omitted entirely, crashing clients like Symfonium during sync.

Ref: https://support.symfonium.app/t/app-gets-stuck-on-syncing-large-database/13004/8

server/subsonic/responses/.snapshots/Responses Child without data should match .JSON

@@ -8,7 +8,8 @@
     "child": [
       {
         "id": "1",
-        "isDir": false
+        "isDir": false,
+        "title": ""
       }
     ],
     "id": "",