chore: remove unnecessary tests and fix test warnings (#236)

Remove tests based on throwing TypeError. They were the cause of the
errors/warnings in IntelliJ and test the PHP type system rather than
Momento validation code. A momento function argument that isn't nullable
throws a TypeError when given null.

Remove tests based on casting null to string. (string)null is equal to
empty string, so those tests were redundant.

Change some of the existing TypeError tests to test empty strings
instead of nulls to test the validation logic for those fields.

Fix two ttl assertions that weren't checking the returned values.

Fix misc. warnings, including unused variables, and extra arguments from
copy/paste errors.