Avoid warning when editing a taxonomy term

When updating a taxonomy term with an empty description we have this error:

Warning: Undefined array key "description" in unique_field_taxonomy_term_form_validate() (line 380 of modules/contrib/unique_field/unique_field.module).
unique_field_taxonomy_term_form_validate()

It needs a small fix when retrieving the description to deal with empty value here

  $description = $form_state->getValues()['description'][0]['value'];