Check for null entries in ConcatenatingMediaSource

We do this in the dynamic case, and I think we've seen
a few GitHub issues where developers do this and don't
understand what they've done wrong (because the failure
comes later).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=163688557

library/core/src/main/java/com/google/android/exoplayer2/source/ConcatenatingMediaSource.java

@@ -57,6 +57,9 @@ public final class ConcatenatingMediaSource implements MediaSource {
    *     {@link MediaSource} instance to be present more than once in the array.
    *     {@link MediaSource} instance to be present more than once in the array.
    */
    */
   public ConcatenatingMediaSource(boolean isRepeatOneAtomic, MediaSource... mediaSources) {
   public ConcatenatingMediaSource(boolean isRepeatOneAtomic, MediaSource... mediaSources) {
+    for (MediaSource mediaSource : mediaSources) {
+      Assertions.checkNotNull(mediaSource);
+    }
     this.mediaSources = mediaSources;
     this.mediaSources = mediaSources;
     this.isRepeatOneAtomic = isRepeatOneAtomic;
     this.isRepeatOneAtomic = isRepeatOneAtomic;
     timelines = new Timeline[mediaSources.length];
     timelines = new Timeline[mediaSources.length];