Subsonic: Fix playlist loading breaking on any invalid track reference

The Subsonic method `getPlaylist` used to fail with an uncaught exception if
the playlist in question contained any references to tracks which do not
exist. This had actually been already fixed once in eae36b8 but then
accidentally reintroduced with e45282e in the v1.8.0.

refs #1087

CHANGELOG.md

@@ -59,8 +59,11 @@
   [#1083](https://github.com/owncloud/music/issues/1083)
 
 ### Fixed
-- Subsonic API: Unhandled exception when attempting to delete a non-existent bookmark
-  [#1071](https://github.com/owncloud/music/issues/1071)
+- Subsonic API:
+  * Unhandled exception when attempting to delete a non-existent bookmark
+    [#1071](https://github.com/owncloud/music/issues/1071)
+  * Method `getPlaylist` failing if the playlist contains any invalid track references (since v1.8.0)
+    [#1087](https://github.com/owncloud/music/issues/1087)
 - Scanning breaking if any out-of-bounds numeric value gets scanned from any audio file
   [#1073](https://github.com/owncloud/music/issues/1073)
 - File and folder selection dialogs not working on NC 27.1.0 and 27.1.1 (workaround for a NC bug which should get fixed in NC 27.1.2)

lib/Db/Track.php

@@ -318,8 +318,8 @@ public function getAdjustedTrackNumber(bool $enablePlaylistNumbering=true) : ?in
 	}
 
 	public function getFileExtension() : string {
-		$parts = \explode('.', $this->getFilename());
-		return \end($parts);
+		$parts = Util::explode('.', $this->getFilename());
+		return empty($parts) ? '' : \end($parts);
 	}
 
 }