Fixed TeamNewPipe/NewPipe#2226.

(in the youtube subscription extractor)
Ignore subscriptions that have an empty title instead of throwing an error: the youtube subscription_manager XML file can sometimes contain those (i.e. deleted channels).

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/extractors/YoutubeSubscriptionExtractor.java

@@ -63,7 +63,11 @@ public class YoutubeSubscriptionExtractor extends SubscriptionExtractor {
             String title = outline.attr("title");
             String xmlUrl = outline.attr("abs:xmlUrl");
 
-            if (title.isEmpty() || xmlUrl.isEmpty()) {
+            if (title.isEmpty()) {
+                continue;
+            }
+
+            if (xmlUrl.isEmpty()) {
                 throw new InvalidSourceException("document has invalid entries");
             }