Handle empty strings in font family names better

Fixes #309.

Author: asamuzaK

lib/properties/fontFamily.js

@@ -45,6 +45,9 @@ function parse(v) {
   });
   const parsedValues = [];
   for (const val of values) {
+    if (!val) {
+      return "";
+    }
     const value = parsers.parsePropertyValue(property, val, {
       caseSensitive: true
     });