AudioEffect: align the interface contract and impl on bad values

The implementation of AudioEffect checks on invalid values
passed to methods drifted over time from the doc. Updates:

  - Add missing checks in the implementation;
  - Update docs;
  - Also, use 'nullptr' instead of 'NULL' for consistency.

Bug: 230640063
Test: atest audioeffect_tests
Change-Id: Ieed8b654b800edc3df0def814db5bfbe8653378f