[Bandcamp] Null-safe url catenation in track playlist

Previously, if no URL was provided due to the track being a preorder or
unpaid track that doesn't have its own public page, we would catenate
the artist URL and null to a nonsensical string.

This invalid URL would also be used to try fetching a thumbnail URL.

The downstream behavior of the NewPipe client is only marginally
improved, as it now no longer throws visible exceptions due to the
missing thumbnails, but still gives an unfriendly error upon clicking
the item that has a `null` URL.

Author: fynngodau

extractor/src/main/java/org/schabi/newpipe/extractor/services/bandcamp/extractors/streaminfoitem/BandcampPlaylistStreamInfoItemExtractor.java

@@ -43,7 +43,12 @@ public String getName() {
 
     @Override
     public String getUrl() {
-        return getUploaderUrl() + track.getString("title_link");
+        final String relativeUrl = track.getString("title_link");
+        if (relativeUrl != null) {
+            return getUploaderUrl() + relativeUrl;
+        } else {
+            return null;
+        }
     }
 
     @Override
@@ -66,7 +71,7 @@ public String getUploaderName() {
     @Nonnull
     @Override
     public List<Image> getThumbnails() throws ParsingException {
-        if (substituteCovers.isEmpty()) {
+        if (substituteCovers.isEmpty() && getUrl() != null) {
             try {
                 final StreamExtractor extractor = service.getStreamExtractor(getUrl());
                 extractor.fetchPage();