Avoid variable from empty module name be empty (#3026)

* Create main.js

* Create _config.js

* Update _config.js

* Update _config.js

* Delete main.js

* Update identifierHelpers.ts

src/utils/identifierHelpers.ts

@@ -24,5 +24,5 @@ export function makeLegal(str: string): string {
 
 	if (startsWithDigit(str) || blacklisted[str]) str = `_${str}`;
 
-	return str;
+	return str || '_';
 }

test/function/samples/avoid-variable-be-empty/_config.js

@@ -0,0 +1,19 @@
+module.exports = {
+	description: 'avoid variable from empty module name be empty',
+	options: {
+		input: '',
+		plugins: [
+			{
+				resolveId (importee) {
+					return '';
+				},
+				load (path) {
+					return 'export default 0;';
+				},
+			}
+		],
+		output: {
+			format: 'cjs'
+		}
+	}
+};