Revert #162 once PHP min supported version is raised to PHP 7.2

[stronk7]

Right now, PHP min supported version is 7.0 and we need to keep it working because other products need to work with 7.0.

Because of that, we had to "downgrade" some recent code to stop using optional hinting in param and return types. That was done in #162.

This issue is about to revert that commit whenever we officially raise codechecker PHP requirements to 7.2 or up.