SDK / exoplayer

This is a temporary fix to prevent NullPointerException exceptions.
Though writing to cache will still fail if the cache folder isn't a real
folder.

There are a few thing we can try:
* The listing might be failing because the cache folder is just created.
We can wait and try again.
* If the cache folder is a regular file, we can try deleting and creating
a folder with the same name.

PiperOrigin-RevId: 234121925