Bugfix: Add safety checks to font_for_string
Bugfix: Add safety checks to font_for_string

* Before font_for_string would fall over if you called it with a missing
  or empty font. Furthermore, it's behavior when called for an empty
  ustring was not intuitively obvious.
* This corrects both behaviors with explicit checks.
* Adds a WARN level log event when it fails out a font for being empty
  or missing, so that a human can correct the situation.

appsembler/edx-certificates,EDUlib/edx-certificates,olexiim/edx-certificates,Stanford-Online/openedx-certificates,ESOedX/edx-certificates,appsembler/edx-certificates,cecep-edu/edx-certificates,eestay/edx-certificates,cecep-edu/edx-certificates,stvstnfrd/edx-certificates,edx/edx-certificates,olexiim/edx-certificates,edx/edx-certificates,stvstnfrd/edx-certificates,marcore/edx-certificates,ESOedX/edx-certificates,EDUlib/edx-certificates,victoria-valdes/edx-certificates,Stanford-Online/openedx-certificates,marcore/edx-certificates,victoria-valdes/edx-certificates,eestay/edx-certificates