Prevent error on default font selection

Seen in console on clean install (using default font selection):

    GET https://fonts.googleapis.com/css?family=%27 400 (Bad Request) 

It is possible for $font_import to still be empty after missing each if statement.

Minor issue in a fantastic theme - thanks for all the effort put into this.
Merge pull request #15 from abovethewater/patch-1

Prevent error on default font selection