[WIP] Strict comparison not null property with null

[ossinkine]

This is the fix for the issue described here phpstan/phpstan#2618.

@ondrejmirtes Could you please give an advice how to add parameter check and how to test this behavior with parameter? If you show examples it would be enough.

And in general, am I moving in the right direction?

[ossinkine]

@ondrejmirtes Could you steer me in the right direction?

[ondrejmirtes]

@ossinkine See for example how other scalar parameters get into MutatingScope constructor, like bool $treatPhpDocTypesAsCertain, and try to add another.

[ondrejmirtes]

As for testing, that's not going to be easy unless you want to run whole PHPStan with exec() and asserting the JSON result. Because the parameter gets into MutatingScope somewhere deep down and it's not easy to override that in an integration test.