Asserting isset twice on maybe-nullable offsets of same array results in error

```php
function getMaybeArray() : ?array {
    if (rand(0, 1)) { return [1, 2, 3]; }
    return null;
}

$arr = [
    'a' => getMaybeArray(),
    'b' => getMaybeArray(),
];
  
if (isset($arr['a']) && isset($arr['b'])) {}
```

Expected: no issue
Actual: Result of && is always true.

https://phpstan.org/r/a8dfa0d7-2df4-4af2-a3fb-6e138c34b36f