SDK / exoplayer

This will help developers self-diagnose issues like Issue: google/ExoPlayer#10392
where the NPE occurs far from the original null value because a field
gets assigned to null.

This change aims to ensure that every stable method on Player,
ExoPlayer and ExoPlayer.Builder that takes a non-null type will fail
with an NPE before returning.

#minor-release

PiperOrigin-RevId: 461846580