set default value to ""

Former-commit-id: cb02dbcc8d637c049f9670932658021d51bb3b17

lib/single-file/single-file-helper.js

@@ -312,7 +312,7 @@ this.singlefile.lib.helper = this.singlefile.lib.helper || (() => {
 		}
 	}
 
-	function normalizeFontFamily(fontFamilyName) {
+	function normalizeFontFamily(fontFamilyName = "") {
 		return removeQuotes(singlefile.lib.vendor.cssUnescape.process(fontFamilyName.trim())).toLowerCase();
 	}