Fix polly media widget settings errors

Problem/Motivation

If you use the module to allow synthesis on a media field, there are some warnings and AJAX funkiness:

• A couple of undefined array key warnings which both come from expecting a language code in the format language-COUNTRY (eg. en-US) which isn't true for arb (standard Arabic):
  • Warning: Undefined array key 1 in Drupal\polly_media\Hook\MediaWidgetAlter->Drupal\polly_media\Hook\{closure}() (line 148 of /var/www/html/modules/polly_media/src/Hook/MediaWidgetAlter.php).
  • Warning: Undefined array key "" in Drupal\polly_media\Hook\MediaWidgetAlter->Drupal\polly_media\Hook\{closure}() (line 151 of /var/www/html/modules/polly_media/src/Hook/MediaWidgetAlter.php).
• If you check Allow content editors to choose the voice and switch between engines, then the Voices label gets repeated - this is because an additional theme wrapper is added to the checkboxes in Checkboxes::preRenderCompositeFormElement() which is run after we've added the wrapping container that gets replaced by AJAX.
• If you uncheck Allow content editors to choose the voice it doesn't hide the Voices checkboxes.

Steps to reproduce

1. Set up a content type with a media field.
2. Go to the form settings and configure the media widget to allow speech synthesis.