Bug 1111903 - [Settings] Refactor Dialog Service with l10n best practice...

...s
it's better to let args be null, than to give them empty {}.

As you can see in https://github.com/mozilla-b2g/gaia/blob/5a84c3259486a589e008a5d5cf5d367984142f9b/shared/js/l10n.js#L1808-L1810 - if we have falsy args, we will avoid setAttribute for them, which is saving DOM mutations.

same for string scenario - args: null.