make sure rule.style.fontFamily is defined

lib/single-file/rules-minifier.js

@@ -101,7 +101,7 @@ this.rulesMinifier = this.rulesMinifier || (() => {
 					stylesheetContent += "}";
 				} else if (rule.selectorText) {
 					stylesheetContent += rule.cssText;
-				} else if (rule.type == CSSRule.FONT_FACE_RULE && rule.style && !unusedFonts.includes(rule.style.fontFamily.trim())) {
+				} else if (rule.type == CSSRule.FONT_FACE_RULE && rule.style && rule.style.fontFamily && !unusedFonts.includes(rule.style.fontFamily.trim())) {
 					stylesheetContent += rule.cssText;
 				} else {
 					stylesheetContent += rule.cssText;