test: add test for null source in ESM loader

Add test cases to verify that loadCJSModule properly handles
null, undefined, and empty string source values by throwing
ERR_INVALID_RETURN_PROPERTY_VALUE instead of triggering
ERR_INTERNAL_ASSERTION.

Tests three scenarios:
- Custom loader returning null source
- Custom loader returning undefined source  
- Custom loader returning empty string source

Refs: #60401

Author: vivek958

test/parallel/test-esm-loader-null-source.js

@@ -0,0 +1,181 @@
+'use strict';
+
+// Test that ESM loader handles null/undefined source gracefully
+// and throws meaningful error instead of ERR_INTERNAL_ASSERTION.
+// Refs: https://github.com/nodejs/node/issues/60401
+
+const common = require('../common');
+const assert = require('assert');
+const { spawnSync } = require('child_process');
+const fixtures = require('../common/fixtures');
+
+// Test case: Loader returning null source for CommonJS module
+// This should throw ERR_INVALID_RETURN_PROPERTY_VALUE, not ERR_INTERNAL_ASSERTION
+{
+  const result = spawnSync(
+    process.execPath,
+    [
+      '--no-warnings',
+      '--input-type=module',
+      '--eval',
+      `
+      import { register } from 'node:module';
+      
+      // Register a custom loader that returns null source
+      const code = 'export function load(url, context, next) {' +
+        '  if (url.includes("test-null-source")) {' +
+        '    return { format: "commonjs", source: null, shortCircuit: true };' +
+        '  }' +
+        '  return next(url);' +
+        '}';
+      
+      register('data:text/javascript,' + encodeURIComponent(code));
+      
+      try {
+        await import('file:///test-null-source.js');
+        console.log('ERROR: Should have thrown');
+        process.exit(1);
+      } catch (err) {
+        // Should throw ERR_INVALID_RETURN_PROPERTY_VALUE, not ERR_INTERNAL_ASSERTION
+        if (err.code === 'ERR_INTERNAL_ASSERTION') {
+          console.log('FAIL: Got ERR_INTERNAL_ASSERTION');
+          process.exit(1);
+        }
+        if (err.code === 'ERR_INVALID_RETURN_PROPERTY_VALUE') {
+          console.log('PASS: Got expected error');
+          process.exit(0);
+        }
+        console.log('ERROR: Got unexpected error:', err.code);
+        process.exit(1);
+      }
+      `,
+    ],
+    { encoding: 'utf8' }
+  );
+
+  const output = result.stdout + result.stderr;
+  
+  // Verify test passed
+  assert.ok(
+    output.includes('PASS: Got expected error'),
+    'Should pass with expected error. Output: ' + output
+  );
+  
+  assert.strictEqual(
+    result.status,
+    0,
+    'Process should exit with code 0. Output: ' + output
+  );
+}
+
+// Test case: Loader returning undefined source
+{
+  const result = spawnSync(
+    process.execPath,
+    [
+      '--no-warnings',
+      '--input-type=module',
+      '--eval',
+      `
+      import { register } from 'node:module';
+      
+      const code = 'export function load(url, context, next) {' +
+        '  if (url.includes("test-undefined-source")) {' +
+        '    return { format: "commonjs", source: undefined, shortCircuit: true };' +
+        '  }' +
+        '  return next(url);' +
+        '}';
+      
+      register('data:text/javascript,' + encodeURIComponent(code));
+      
+      try {
+        await import('file:///test-undefined-source.js');
+        console.log('ERROR: Should have thrown');
+        process.exit(1);
+      } catch (err) {
+        if (err.code === 'ERR_INTERNAL_ASSERTION') {
+          console.log('FAIL: Got ERR_INTERNAL_ASSERTION');
+          process.exit(1);
+        }
+        if (err.code === 'ERR_INVALID_RETURN_PROPERTY_VALUE') {
+          console.log('PASS: Got expected error');
+          process.exit(0);
+        }
+        console.log('ERROR: Got unexpected error:', err.code);
+        process.exit(1);
+      }
+      `,
+    ],
+    { encoding: 'utf8' }
+  );
+
+  const output = result.stdout + result.stderr;
+  
+  assert.ok(
+    output.includes('PASS: Got expected error'),
+    'Should pass with expected error for undefined. Output: ' + output
+  );
+  
+  assert.strictEqual(
+    result.status,
+    0,
+    'Process should exit with code 0. Output: ' + output
+  );
+}
+
+// Test case: Loader returning empty string source
+{
+  const result = spawnSync(
+    process.execPath,
+    [
+      '--no-warnings',
+      '--input-type=module',
+      '--eval',
+      `
+      import { register } from 'node:module';
+      
+      const code = 'export function load(url, context, next) {' +
+        '  if (url.includes("test-empty-source")) {' +
+        '    return { format: "commonjs", source: "", shortCircuit: true };' +
+        '  }' +
+        '  return next(url);' +
+        '}';
+      
+      register('data:text/javascript,' + encodeURIComponent(code));
+      
+      try {
+        await import('file:///test-empty-source.js');
+        console.log('ERROR: Should have thrown');
+        process.exit(1);
+      } catch (err) {
+        if (err.code === 'ERR_INTERNAL_ASSERTION') {
+          console.log('FAIL: Got ERR_INTERNAL_ASSERTION');
+          process.exit(1);
+        }
+        if (err.code === 'ERR_INVALID_RETURN_PROPERTY_VALUE') {
+          console.log('PASS: Got expected error');
+          process.exit(0);
+        }
+        console.log('ERROR: Got unexpected error:', err.code);
+        process.exit(1);
+      }
+      `,
+    ],
+    { encoding: 'utf8' }
+  );
+
+  const output = result.stdout + result.stderr;
+  
+  assert.ok(
+    output.includes('PASS: Got expected error'),
+    'Should pass with expected error for empty string. Output: ' + output
+  );
+  
+  assert.strictEqual(
+    result.status,
+    0,
+    'Process should exit with code 0. Output: ' + output
+  );
+}
+
+console.log('All tests passed!');