minor refactoring

lib/single-file/single-file-core.js

@@ -732,10 +732,8 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 						if (options.compressCSS) {
 							const removedRules = [];
 							for (let cssRule = stylesheet.children.head; cssRule; cssRule = cssRule.next) {
-								if (cssRule.data.type == "Raw") {
-									if (cssRule.data.value.trim().startsWith("//")) {
-										removedRules.push(cssRule);
-									}
+								if (cssRule.data.type == "Raw" && cssRule.data.value && cssRule.data.value.trim().startsWith("//")) {
+									removedRules.push(cssRule);
 								}
 							}
 							removedRules.forEach(cssRule => stylesheet.children.remove(cssRule));