NoNullPropertyInitializationFixer - fix static properties as well

[HypeMC]

The NoNullPropertyInitializationFixer doesn't fix properties with a default null value if they are static. This PR fixes the issue.

I added some extra logic to check if the token is inside a class, but not a method to avoid modifying static variables:

class Foo
{
    static $bar = null; // This is fixed

    public function foo()
    {
        static $baz = null; // This is skipped
    }
}

[coveralls]

Coverage increased (+0.01%) to 91.569% when pulling f33e72f on HypeMC:static-property-with-null into 15ddc20 on FriendsOfPHP:2.19.

[kubawerlos]

@julienfalque
if this is enhancement ➡️ target branch is incorrect,
if this is a bug ➡️ label is incorrect.

[julienfalque]

if this is enhancement -> target branch is incorrect

Is it? Until now I thought we considered enhancements as something in between bugs and new features, e.g. something a rule does not do yet but should have done from the beginning, like this PR.

[keradus]

since a while i was move toward "enhancements === features". but yeah, we have no official, clear policy. I'm happy to merge to both lines.

[keradus]

Thank you @HypeMC.