ffixed issue with inavlid rules

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

@@ -274,7 +274,9 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 		removedNodes.pop();
 		removedNodes.forEach(node => cssRule.block.children.remove(node));
 		const srcDeclaration = cssRule.block.children.filter(node => node.property == "src").tail;
-		srcDeclaration.data.value = cssTree.parse(fontInfo.map(fontSource => fontSource.src).join(","), { context: "value" });
+		if (srcDeclaration) {
+			srcDeclaration.data.value = cssTree.parse(fontInfo.map(fontSource => fontSource.src).join(","), { context: "value" });
+		}
 	}
 
 	function filterUnusedFonts(rules, declaredFonts, unusedFonts, filteredUsedFonts, docContent) {
@@ -435,7 +437,7 @@ this.fontsMinifier = this.fontsMinifier || (() => {
 		]);
 	}
 
-	function getFontFamily(string) {
+	function getFontFamily(string = "") {
 		string = string.toLowerCase().trim();
 		if (string.match(REGEXP_SIMPLE_QUOTES_STRING)) {
 			string = string.replace(REGEXP_SIMPLE_QUOTES_STRING, "$1");