fix regression of saved date values

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

@@ -1508,7 +1508,7 @@ this.singlefile.lib.core = this.singlefile.lib.core || (globalThis => {
 				}
 			}
 
-			async function evalDate(date, prefix) {
+			async function evalDate(date, prefix = "") {
 				if (date) {
 					template = await evalTemplateVariable(template, prefix + "datetime-iso", () => date.toISOString(), dontReplaceSlash, options.filenameReplacementCharacter);
 					template = await evalTemplateVariable(template, prefix + "date-iso", () => date.toISOString().split("T")[0], dontReplaceSlash, options.filenameReplacementCharacter);