`checkNotNull` should be avoided where possible.
This change adds `@EnsuresNonNull` or `@EnsuresNonNullIf` to configuration methods for fields they initialize.
`checkNotNull` is now avoided for the `@MonotonicNonNull` formats by adding `@RequiresNonNull` annotations.
`checkNotNull` is now avoided for the encoder and decoder in `feedMuxerFromEncoder()`, `feedEncoderFromDecoder()`, `feedDecoderFromInput()`, etc. by creating local variables for `encoder` and `decoder` in `render` after the configuration method calls and passing these as non-null parameters.
PiperOrigin-RevId: 405893824
