Fix null safety error in inspector service extensions taking a variable number of args. (#68661)

* Fix null safety error in inspector service extensions taking variable numbers of args.

Fixes https://github.com/flutter/flutter/issues/68627.
Also fix all inspector tests that do not rely on golden image functionality
so that they run with --enable-experiment=non-nullable. This verifies there
is test coverage to ensure there isn't a regression.

* Remove uses of dynamic.