Fix getSubscriberCount() for 0 subscribers

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

@@ -177,8 +177,12 @@ public class YoutubeChannelExtractor extends ChannelExtractor {
                 throw new ParsingException("Could not get subscriber count", e);
             }
         } else {
-            // If the element is null, the channel have the subscriber count disabled
-            return -1;
+            // If there's no subscribe button, the channel has the subscriber count disabled
+            if (initialData.getObject("header").getObject("c4TabbedHeaderRenderer").getObject("subscribeButton") == null) {
+                return -1;
+            } else {
+                return 0;
+            }
         }
     }