Allow negated assignment in else

`TypeTest::testNegatedAssignmentInIf()`:

```php
if (!($row = (rand(0, 10) ? [5] : null))) {
    // do nothing
}  else {
    echo $row[0];
}
```

This code emits a `MixedArrayOffset` issue because the type the if condition evaluates to `empty`, and negating `empty` produces `mixed`.
