Small change to isolate NPE

We have NPE reports from the line mpris.fetchedAlbumArt(url.toString()).
I'm not sure if it's because mpris is null or url is null. Spliting in two
lines to see where the crash actually happens.

src/org/kde/kdeconnect/Plugins/MprisPlugin/AlbumArtCache.kt

@@ -384,7 +384,8 @@ internal object AlbumArtCache {
 
             //So notify the mpris plugins of the fetched art
             for (mpris in registeredPlugins) {
-                mpris.fetchedAlbumArt(url.toString())
+                val stringUrl = url.toString()
+                mpris.fetchedAlbumArt(stringUrl)
             }
         } else {
             //Mark the fetch as failed in the memory cache