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
(cherry picked from commit e1fde5d530a43b64f6e0603843ccafe7692e8453)
d5e0c7597a