set filename to "" by default

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

@@ -414,7 +414,7 @@ this.SingleFileCore = this.SingleFileCore || (() => {
 				this.stats.set("processed", "HTML bytes", contentSize);
 				this.stats.add("discarded", "HTML bytes", size - contentSize);
 			}
-			let filename = await ProcessorHelper.evalTemplate(this.options.filenameTemplate, this.options, content);
+			let filename = await ProcessorHelper.evalTemplate(this.options.filenameTemplate, this.options, content) || "";
 			filename = filename.replace(/[~\\?%*:|"<>\x00-\x1f\x7F]+/g, "_"); // eslint-disable-line no-control-regex
 			filename = filename.replace(/\.\.\//g, "").replace(/^\/+/, "").replace(/\/+/g, "/").replace(/\/$/, "");
 			if (!this.options.backgroundSave) {