Remove the usage of "null" locale to symbolize Locale.ROOT

[claesrosell]

Issue #16 replaced the usage of UIUtils.ROOT with Locale.ROOT, however, it looks like "null" is used in various places of Babel Plugins to symbolize the ROOT locale. In some places used as key in maps. We may need a two step aproch where both values are allowed at first, like:

void someMethod(Locale locale) {
      Locale localeToUse = locale == null ? Locale.ROOT : locale;
      String value = this.hashMap.get(localeToUse);
      ...
}