Don't require source entity type to be specified in querystring if it is configured

The test fails were a bit weird, seems like the querystring source plugin is running when webform submissions are being viewed, but in those cases there is no upcast webform route parameter to get the setting from.

We need to document the steps required to reproduce this issue.

Added steps to reproduce

I tweaked the MR to be very simple enhancement.

I reverted my two commits. I am baffled why all those tests broken.

Below is the code snippet I was trying to use

    // Get and check source entity type.
    $source_entity_type = $this->request->query->get('source_entity_type');
    // If the source entity type is not defined, attempt to get the
    // source entity type from the webform's settings.
    if (!$source_entity_type && $webform->getSetting('form_prepopulate_source_entity')) {
      $source_entity_type = $webform->getSetting('form_prepopulate_source_entity_type');
    }
    if (!$source_entity_type || !$this->entityTypeManager->hasDefinition($source_entity_type)) {
      return NULL;
    }

See #5. In some circumstances the webform parameter is not upcast by the route.

I am open to merging this AS-IS