Appending to empty arrays creates a mixed array

```php
/** @return int[] */
function foo() : array {
  $arr = [];
  $arr[] = new stdClass;
  return $arr;
}
```
https://phpstan.org/r/ea6c23403e832684333cea82e06c7d04

The return type `int[]` is wrong here, but PHPStan cannot see it. 

