macosx/library: don't try to show thumbnails for media where the generation failed

Just because the generation was started, does not mean it succeeded, so we cannot trust the state but need to check for the URL's validity.

modules/gui/macosx/library/VLCLibraryDataTypes.h

@@ -169,7 +169,7 @@ extern const long long int VLCMediaLibraryMediaItemDurationDenominator;
 @property (readonly) time_t lastPlayedDate;
 @property (readonly) NSString *title;
 
-@property (readonly) NSString *smallArtworkMRL;
+@property (readonly, nullable) NSString *smallArtworkMRL;
 
 @property (readonly) BOOL smallArtworkGenerated;
 @property (readonly) BOOL favorited;

modules/gui/macosx/library/VLCLibraryDataTypes.m

@@ -295,8 +295,12 @@ NSString *VLCMediaLibraryMediaItemLibraryID = @"VLCMediaLibraryMediaItemLibraryI
         _playCount = p_mediaItem->i_playcount;
         _lastPlayedDate = p_mediaItem->i_last_played_date;
         _title = toNSStr(p_mediaItem->psz_title);
-        _smallArtworkMRL = toNSStr(p_mediaItem->thumbnails[VLC_ML_THUMBNAIL_SMALL].psz_mrl);
-        _smallArtworkGenerated = p_mediaItem->thumbnails[VLC_ML_THUMBNAIL_SMALL].b_generated;
+        _smallArtworkGenerated = p_mediaItem->thumbnails[VLC_ML_THUMBNAIL_SMALL].psz_mrl != NULL;
+        if (_smallArtworkGenerated) {
+            _smallArtworkMRL = toNSStr(p_mediaItem->thumbnails[VLC_ML_THUMBNAIL_SMALL].psz_mrl);
+        } else {
+            _smallArtworkMRL = nil;
+        }
         _favorited = p_mediaItem->b_is_favorite;
 
         switch (p_mediaItem->i_subtype) {