Replace deprecated getClass function with getType

[MaximVanhove]

This change removes the deprecated warning for ReflectionParameter::getClass in PHP 8 and adds support for nullable arguments.

This change was taken from the 3.x branch.

[philipobenito]

Will have to remind myself why this was removed in the first place, there was a good reason but can't remember what it was.

Planning some merges and releases next week so will look then.