AssertEmptyNullableObjectToAssertInstanceofRector phpunit rule converting assertNull into assertInstanceOf, which isn't more narrow

[jaydiablo]

Bug Report

Subject	Details
Rector version	2.6.3

With these config settings in rector.php:

->withComposerBased(phpunit: true)
    ->withPreparedSets(phpunitCodeQuality: true)

We're seeing rector change $this->assertNull($object); into something like $this->assertNotInstanceOf(someClass::class, $object) which, IMO, defeats the purpose of this rule (I believe it's trying to narrow phpunit assertions), as this widens the assertion quite a bit.

I do agree that the opposite behavior this rule does (converts $this->assertNotNull($object) into $this->assertInstanceOf(someClass::class, $object) is helpful.

Is this the intended behavior of this rule (specifically changing assertNull)? If so, is it possible to skip just the assertNull -> assertNotInstanceOf conversion without skipping the entire rule?

Minimal PHP Code Causing Issue

https://getrector.com/demo/d97c4200-16c1-452a-8b45-b4f6b2d6a95c

Expected Behaviour

I'd expect rector to ignore the assertNull assertion, since that seems like a more narrow type than assertNotInstanceOf in this contrived example (but we saw this with real world code as well).

[TomasVotruba]

Good point 👍 care to PR?