These fields can't be used if `drm_uuid` isn't set. Make that case throw
an exception, so it's obvious to a developer what's wrong.
Most of the fields 'obviously' need `drm_uuid` to be set, but it's not
obvious for `drm_session_for_clear_content` (because it might be
reasonable to assume it's possible to play clear content without
specifying a UUID). This tripped me up in
https://github.com/google/ExoPlayer/issues/8842#issuecomment-833659808.
PiperOrigin-RevId: 397328556
