Implement some methods in PlaylistExtractor

This will prevent their override in each child class where the values corresponding to the methods could not be extracted.

Author: AudricV

extractor/src/main/java/org/schabi/newpipe/extractor/playlist/PlaylistExtractor.java

@@ -8,24 +8,45 @@
 
 import javax.annotation.Nonnull;
 
+import static org.schabi.newpipe.extractor.utils.Utils.EMPTY_STRING;
+
 public abstract class PlaylistExtractor extends ListExtractor<StreamInfoItem> {
 
-    public PlaylistExtractor(StreamingService service, ListLinkHandler linkHandler) {
+    public PlaylistExtractor(final StreamingService service, final ListLinkHandler linkHandler) {
         super(service, linkHandler);
     }
 
-    public abstract String getThumbnailUrl() throws ParsingException;
-    public abstract String getBannerUrl() throws ParsingException;
-
     public abstract String getUploaderUrl() throws ParsingException;
     public abstract String getUploaderName() throws ParsingException;
     public abstract String getUploaderAvatarUrl() throws ParsingException;
     public abstract boolean isUploaderVerified() throws ParsingException;
 
     public abstract long getStreamCount() throws ParsingException;
 
-    @Nonnull public abstract String getSubChannelName() throws ParsingException;
-    @Nonnull public abstract String getSubChannelUrl() throws ParsingException;
-    @Nonnull public abstract String getSubChannelAvatarUrl() throws ParsingException;
+    @Nonnull
+    public String getThumbnailUrl() throws ParsingException {
+        return EMPTY_STRING;
+    }
+
+    @Nonnull
+    public String getBannerUrl() throws ParsingException {
+        // Banner can't be handled by frontend right now.
+        // Whoever is willing to implement this should also implement it in the frontend.
+        return EMPTY_STRING;
+    }
+
+    @Nonnull
+    public String getSubChannelName() throws ParsingException {
+        return EMPTY_STRING;
+    }
 
+    @Nonnull
+    public String getSubChannelUrl() throws ParsingException {
+        return EMPTY_STRING;
+    }
+
+    @Nonnull
+    public String getSubChannelAvatarUrl() throws ParsingException {
+        return EMPTY_STRING;
+    }
 }