ensure template is defined

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

@@ -1549,7 +1549,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 
 		static async evalTemplateVariable(template, variableName, valueGetter, dontReplaceSlash) {
 			const replaceRegExp = new RegExp("{\\s*" + variableName + "\\s*}", "g");
-			if (template.match(replaceRegExp)) {
+			if (template && template.match(replaceRegExp)) {
 				let value = await valueGetter();
 				if (!dontReplaceSlash) {
 					value = value.replace(/\/+/g, "_");