Fixed issue with translation loading without theme

Fixed issue with translation loading without theme

System was using the empty state return from theme_path,
when no theme was configured, for loading in languages
which would result in the root path being looked up upon.

This changes the theme_path helper to return null in cases a theme
is not configured instead of empty string to help prevent assumed
return path will be legitimate, and to help enforce error case
handling.

For #2836