Update YoutubeStreamExtractor.java

To fix ``java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String[] java.lang.String.split(java.lang.String)' on a null object reference``

Author: litetex

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

@@ -1049,7 +1049,16 @@ public List<Frameset> getFrames() throws ExtractionException {
                 storyboardsRenderer = storyboards.getObject("playerStoryboardSpecRenderer");
             }
 
-            final String[] spec = storyboardsRenderer.getString("spec").split("\\|");
+            if (storyboardsRenderer == null) {
+                return new ArrayList<>();
+            }
+
+            final String storyboardsRendererSpec = storyboardsRenderer.getString("spec");
+            if (storyboardsRendererSpec == null) {
+                return new ArrayList<>();
+            }
+
+            final String[] spec = storyboardsRendererSpec.split("\\|");
             final String url = spec[0];
             final ArrayList<Frameset> result = new ArrayList<>(spec.length - 1);