test: use index.js if package.json "main" is empty

Verify that the module loader uses index.js when the "main" property of
package.json is the empty string.

Refs: #32013

PR-URL: #32040
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/fixtures/require-empty-main/index.js

@@ -0,0 +1,2 @@
+'use strict';
+module.exports = 42;

test/fixtures/require-empty-main/package.json

@@ -0,0 +1 @@
+{"main":""}

test/parallel/test-require-empty-main.js

@@ -0,0 +1,25 @@
+'use strict';
+require('../common');
+
+// A package.json with an empty "main" property should use index.js if present.
+// require.resolve() should resolve to index.js for the same reason.
+//
+// In fact, any "main" property that doesn't resolve to a file should result
+// in index.js being used, but that's already checked for by other tests.
+// This test only concerns itself with the empty string.
+
+const assert = require('assert');
+const path = require('path');
+const fixtures = require('../common/fixtures');
+
+const where = fixtures.path('require-empty-main');
+const expected = path.join(where, 'index.js');
+
+test();
+setImmediate(test);
+
+function test() {
+  assert.strictEqual(require.resolve(where), expected);
+  assert.strictEqual(require(where), 42);
+  assert.strictEqual(require.resolve(where), expected);
+}