[AVOD] Added if empty checks for streamKeys.

DashMediaSource is created with Manifest data and streamKeys can be empty given that streamKeys are only created out of MediaItem inside DashMediaSource, and this particular implementation don't extract them out of MediaItem properties.

Tested=Verified playback works.
PiperOrigin-RevId: 298550021

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -258,7 +258,7 @@ public final class DashMediaSource extends BaseMediaSource {
      */
     public DashMediaSource createMediaSource(DashManifest manifest) {
       Assertions.checkArgument(!manifest.dynamic);
-      if (streamKeys != null) {
+      if (streamKeys != null && !streamKeys.isEmpty()) {
         manifest = manifest.copy(streamKeys);
       }
       return new DashMediaSource(