[CodeQuality] Handle crash on return array constant on ExplicitReturnNullRector

[samsonasik]

Given the following code:

class SkipReturnArrayConstant
{
    public static function provideClearSession()
    {
        return [
            ['Default', ['Foo' => ['foo' => 'fooValue']]],
            [null, []],
        ];
    }
}

got crash:

There was 1 error:

1) Rector\Tests\CodeQuality\Rector\ClassMethod\ExplicitReturnNullRector\ExplicitReturnNullRectorTest::test with data set #1 ('/Users/samsonasik/www/rector-...hp.inc')
Rector\Exception\Reflection\MissingPrivatePropertyException: Property "$keyType" was not found in "PHPStan\Type\Constant\ConstantArrayType" class

This PR fix it.

[samsonasik]

Fixed 🎉