Avoid storing empty third party settings when a form is added through hook_field_widget_third_party_settings_form

+1 for this. My config is riddled with "empty" config like this.

People looking for a temporary workaround, this one seems to work for me:

/**
 * Implements hook_field_widget_third_party_settings_form().
 */
function mymodule_ux_field_widget_third_party_settings_form(WidgetInterface $plugin, FieldDefinitionInterface $field_definition, $form_mode, array $form, FormStateInterface $form_state) {
    $element = [];

    // Add a "nested_hide_level" checkbox to the settings form.
    $element['mysetting'] = [
      '#type' => 'checkbox',
      '#title' => $this->t('My setting'),
      '#default_value' => $plugin->getThirdPartySetting('mymodule', 'mysetting'),
      '#required' => FALSE,
      '#element_validate' => ['_mymodule_validate_mysetting'],
    ];

    return $element;
}

/**
 * Validates mysetting.
 */
function _mymodule_validate_mysetting(&$element, FormStateInterface $form_state, &$complete_form) {
  if (empty($element['#value'])) {
    $form_state->unsetValue(array_slice($element['#parents'], 0, -1));
  }
}