fix undefined value

Former-commit-id: 81d6995ff87c6596d70deb29898c669512027db5

lib/single-file/modules/css-fonts-minifier.js

@@ -300,8 +300,8 @@ this.singlefile.lib.modules.fontsMinifier = this.singlefile.lib.modules.fontsMin
 	}
 
 	function testUnicodeRange(docContent, unicodeRange) {
-		unicodeRange = singlefile.lib.helper.removeQuotes(unicodeRange);
 		if (unicodeRange) {
+			unicodeRange = singlefile.lib.helper.removeQuotes(unicodeRange);
 			const unicodeRanges = unicodeRange.split(REGEXP_COMMA);
 			let invalid;
 			const result = unicodeRanges.filter(rangeValue => {