Fix apply() to work with an empty string.

Change-Id: Ib7f7c9d9a0d99b3768cea4f7c77f8477c3421bae

lib/jquery.webfonts.js

@@ -70,8 +70,12 @@
 		apply : function(fontFamily, $element) {
 			$element = $element || this.$element;
 			var fontStack = this.options.fontStack.slice(0);
+			// Loading an empty string is pointless.
+			// Putting an empty string into a font-family list doesn't work with jQuery.css().
+			if ( fontFamily ) {
 				this.load(fontFamily);
 				fontStack.unshift(fontFamily);
+			}
 			$element.css('font-family', fontStack.join() );
 			$element.find('textarea, input').css('font-family', fontStack.join());
 		},