VLCPlaybackServiceDelegate: mediaForPlayingMedia: Fix nullability

Set correct nullability for the media parameter since it can be nil.

If media was nil, it would lead to a crash in Swift -> AudioMiniPlayer

Sources/MiniPlayer/AudioMiniPlayer.swift

@@ -118,7 +118,11 @@ extension AudioMiniPlayer {
         mediaService.savePlaybackState(from: playbackService)
     }
 
-    func media(forPlaying media: VLCMedia) -> VLCMLMedia? {
+    func media(forPlaying media: VLCMedia?) -> VLCMLMedia? {
+        guard let media = media else {
+            return nil
+        }
+
         return mediaService.fetchMedia(with: media.url)
     }
 }

Sources/VLCPlaybackService.h

@@ -39,7 +39,7 @@ extern NSString *const VLCPlaybackServicePlaybackPositionUpdated;
 @protocol VLCPlaybackServiceDelegate <NSObject>
 #if TARGET_OS_IOS
 - (void)savePlaybackState:(VLCPlaybackService *)playbackService;
-- (VLCMLMedia *_Nullable)mediaForPlayingMedia:(VLCMedia *)media;
+- (VLCMLMedia *_Nullable)mediaForPlayingMedia:(nullable VLCMedia *)media;
 #endif
 @optional
 - (void)playbackPositionUpdated:(VLCPlaybackService *)playbackService;