set outputDirectory option to "" by default

cli/single-file-cli-api.js

@@ -230,7 +230,7 @@ async function capturePage(options) {
 }
 
 function getFilename(filename, options, index = 1) {
-	let outputDirectory = options.outputDirectory;
+	let outputDirectory = options.outputDirectory || "";
 	if (outputDirectory && !outputDirectory.endsWith("/")) {
 		outputDirectory += "/";
 	}