Fixes t helper when used in controllers, components etc.

If there are no interpolation values for a translation (e.g. just `this.t('name')`) `return fmt(result, readArray(values));` fails as `values` is undefined. When called as a Handlebars helper, values are passed as an empty array when the argument is missing so that works.
We shouldn't reassign the argument name (it is harder to debug).