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
Allows someone using the theme system to configure the social driver
before a redirect action occurs, by passing a callback as an additional
param to the theme 'addSocialDriver' method.
