fix: the empty string importee should not always be treated as extern…

…al module id (#2733)

* Update mergeOptions.ts

* Add test for using empty string as virtual module name

src/utils/mergeOptions.ts

@@ -168,7 +168,7 @@ function addUnknownOptionErrors(
 
 function getCommandOptions(rawCommandOptions: GenericConfigObject): GenericConfigObject {
 	const command = { ...rawCommandOptions };
-	command.external = (rawCommandOptions.external || '').split(',');
+	command.external = rawCommandOptions.external ? rawCommandOptions.external.split(',') : [];
 
 	if (rawCommandOptions.globals) {
 		command.globals = Object.create(null);

test/function/samples/empty-string-as-module-name/_config.js

@@ -0,0 +1,15 @@
+module.exports = {
+	description: 'allows using empty string as a valid module name',
+	options: {
+		external: () => false,
+		plugins: [
+			{
+				resolveId(importee, importer) {
+					if (importee === '') {
+						return importer + '#.js';
+					}
+				}
+			}
+		]
+	}
+};

test/function/samples/empty-string-as-module-name/main.js

@@ -0,0 +1,2 @@
+import { importee } from '';
+assert.strictEqual(importee, 'xxx');

test/function/samples/empty-string-as-module-name/main.js#.js

@@ -0,0 +1 @@
+export var importee = 'xxx';