[Fix #1620] Fix false positives in Rails/StrongParametersExpect

[koic]

This PR fixes false positives in Rails/StrongParametersExpect when params[:key] is followed by a type-check method such as is_a?, kind_of?, or instance_of?.

These methods are inherited by NilClass from BasicObject/Object, so calling them on a missing parameter simply returns false instead of raising. Callers using params[:key].is_a?(...) are intentionally treating the parameter as optional, and rewriting the expression to params.expect(:key).is_a?(...) changes that semantics because expect raises ActionController::ParameterMissing when the parameter is absent.

Since this shares the same "safe to call on nil" rationale as the existing NIL_SAFE_CONVERSION_METHODS (to_a, to_f, to_h, to_i, to_s), the constant is renamed to NIL_SAFE_METHODS and the type-check methods are merged into it rather than introducing a separate constant.

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