Allow null DataSink.Factory in CacheDataSourceFactory

CacheDataSource allows null DataSink. Do the same in CacheDataSourceFactory.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=149750696

library/src/main/java/com/google/android/exoplayer2/upstream/cache/CacheDataSourceFactory.java

@@ -68,7 +68,8 @@ public final class CacheDataSourceFactory implements DataSource.Factory {
   public CacheDataSource createDataSource() {
     return new CacheDataSource(cache, upstreamFactory.createDataSource(),
         cacheReadDataSourceFactory.createDataSource(),
-        cacheWriteDataSinkFactory.createDataSink(), flags, eventListener);
+        cacheWriteDataSinkFactory != null ? cacheWriteDataSinkFactory.createDataSink() : null,
+        flags, eventListener);
   }
 
 }