Refactor FormTestClickedButtonForm::buildForm

Problem/Motivation

In 📌 Fix strict type errors in test modules Needs work it was suggested we open a follow up to refactor FormTestClickedButtonForm::buildForm.

Steps to reproduce

See core/modules/system/tests/modules/form_test/src/Form/FormTestClickedButtonForm.php

Proposed resolution

I think this warrants a follow-up to make this less magic.
We can use array_filter to trim out the nulls and we can pass the actual types instead of magic s/b/i and r

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet