Fix 'undefined variable' PHPStan L0 errors

Note: The code here wasn't quite as dead as it seemed. It's a copy from the parent:

    if (!empty($def['join_id'])) {
      $id = $def['join_id'];
    }
    else {
      $id = 'standard';
    }

For EntityReverse, $def == $alias as there are two tables, this wasn't adjusted when it was originally copied from the parent. In practice, this never worked and it was never needed to use a different join plugin than standard, so removing it seems fine.

Found this because I was trying to understand the copied code added in 🐛 Views doesn't recognize relationship to host Needs review