allow upper case NULL class constant for PSR-2

PSR-2 states that: 

> The PHP constants true, false, and null MUST be in lower case.

But PSR-1 states:

> Class constants MUST be declared in all upper case with underscore separators.

This PR allows class constants named `NULL` to be uppercase, while still ensuring non-class constant usages of `null` are lower cased:

```php
class MyClass
{
    const NULL = null;
}

var_dump(MyClass::NULL);
var_dump(null);
``` 

Current phpcs throws an error on the class constant.