[BUG] "Method…should return Generator but returns false"

This is a false positive, because returning `false` from a method which `yield`s still returns a `Generator`, see:
```php
<?php

function foo(): Generator
{
    return false;
    yield "yolo";
}

var_dump(foo());
```
Output:
```
object(Generator)#1 (0) { }
```
See also https://3v4l.org/VcSpa