Do not process empty batches.

[idimopoulos]

Hi @jhedstrom
I have had a weird issue with one of my behat test so before anything, this can be fixed by filtering users anew at https://github.com/jhedstrom/drupalextension/blob/main/src/Drupal/DrupalExtension/Context/RawDrupalContext.php#L266 before we call for the process batch.

I managed to reproduce it in behat by simply creating a basic test

@api
Feature: Ensure user cancellation is not breaking behat.

  Scenario: User cancels the account
    Given users:
      | name | pass |
      | test | test |
    Given I delete the user "test"

What happens, is that the cleanUsers tries to clean users by calling user_cancel. In /core/modules/user/user.module:614 of Drupal, a check occurs for whether the user actually exists and just throws an error that we are trying to delete a user that does not exist.

However, in RawDrupalContext, the ::processCallback is called and the Drupal8 driver just does

public function processBatch() {
    $this->validateDrupalSite();
    $batch =& batch_get();
    $batch['progressive'] = FALSE;
    batch_process();
  }

Because the batch from batch_get is returned empty, the progressive key is added to an empty array and the process is called. Then the batch fails with an error Warning: Undefined array key "sets" in /var/www/html/web/core/includes/form.inc line 925 and the behat test partially succeeds.

As said above, this can be fixed here with this PR, or in RawDrupalContext from drupalextension or in both.

[idimopoulos]

Please, note, as I forgot to mention, that the dummy step that I created "I delete the user 'test'", simply does a$user->delete() but it is a fake step that represents ANY way that a user can be deleted while the test is running. It can be through the UI by the user themselves (with email verification) - or by an admin, or by some cron operation, or by some spam mechanism, or by any way while the test is running and is outside the scope of the behat API.