getDefaultValue(customSelect) {
    const val = customSelect.getAttribute('value');
    if (val === null) {
      return '';
    }
    const firstChar = val[0];
    if (firstChar === undefined) {
      return '';
    } else if (firstChar === '[') {
      return JSON.parse(val);
    }
    return val;
  }
Get default value from value="" attribute
which might be a string representing a single selected option value
or a JSON array representing selected options in multiple select

This attribute has highest priority over aria-selected which will be updated in syncUI()
If value is empty string or no value attribute found then 1st option is selected

@param customSelect
@returns {String|Array}