[13.x] Ensure assertModelMissing and assertModelExists dont silently pass

[jackbayliss]

Noticed I missed checking its not an empty array because... https://3v4l.org/qKWot#v8.5.3 is true

 // Currently this would pass, even tho it shouldnt :o 
 $user = User::factory()->create();                                                                                                                                                                      
 $this->assertModelMissing($user);

I've also added some tests to ensure it never catches a poor soul again. As these weren't tested afaik!

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