Add a condition to avoid entering nil values into dictionaries

[gskbyte]

Fixes a potential bug found by the static analyser that could occur when we are trying to set an unknown font.

[coveralls]

Coverage decreased (-0.42%) when pulling 21d6714 on fix_possible_crash_on_unknown_fonts into e5c835f on master.

[pietbrauer]

If you could add a test for that, that would be nice :)