ensure options.doc is defined (bg save)

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

@@ -67,7 +67,7 @@ this.singlefile.lib.modules.fontsMinifier = this.singlefile.lib.modules.fontsMin
 			});
 			workStyleElement.remove();
 			docContent += doc.body.innerText;
-			if (getComputedStyle) {
+			if (getComputedStyle && options.doc) {
 				fontsInfo.used = fontsInfo.used.map(fontNames => fontNames.map(familyName => {
 					const matchedVar = familyName.match(/^var\((--.*)\)$/);
 					if (matchedVar && matchedVar[1]) {