Fix t without strings

Let's be sure that the t() functions receive a string.

This is mandatory because if we call t() with an undefined value, and if the current locale is not en_US, the execution halts with the error message `Illegal offset type`.

I encountered this while browsing to http://mysite/index.php/dashboard/system/basics/editor, where `$pageTitle` is undefined in [concrete/themes/dashboard/view.php](https://github.com/concrete5/concrete5-5.7.0/blob/93336287feb651792ec3d3c83229719ae550a172/web/concrete/themes/dashboard/view.php#L6) (and that's another problem :wink:)