Modify AssertHasKey and AssertHasNotKey to work with null value

Currently the new Assertion methods only check for a key
with `isset` and that a value is present (not null).

This change modifies the `isset` to use `array_key_exists`
to verify a key is in an array regardless of the value of
they key. This ensure if the value is null, and they key
is there the test passes.