Don't try to load a font CSS file if none is specified. #35
Description
If someone omits the "font" attribute from the Wordpress shortcode, the plugin specifies
font: ""
in the configuration JSON, leading to a 404 trying to load something like http://knightlab.northwestern.edu/wp-content/plugins/knight-lab-timelinejs/css/themes/font/.css?2.24
we should try to omit the empty string configuration parameter when there is no font. (And similar for any other optional configuration properties)
(see this Wordpress Plugin Support ticket)