fix: support export default null (#22)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Improved compatibility with CommonJS modules simulating ES modules by
updating condition checks when importing modules.

- **Tests**
- Added test cases to support scenarios where modules have `null` as the
default export.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

src/import.ts

@@ -35,7 +35,7 @@ export async function importModule(filepath: string, options?: ImportModuleOptio
     // commonjs
     obj = require(moduleFilePath);
     debug('[importModule] require %o => %o', filepath, obj);
-    if (obj?.__esModule === true && obj?.default) {
+    if (obj?.__esModule === true && 'default' in obj) {
       // 兼容 cjs 模拟 esm 的导出格式
       // {
       //   __esModule: true,
@@ -72,7 +72,7 @@ export async function importModule(filepath: string, options?: ImportModuleOptio
       obj = obj.default;
     }
     if (options?.importDefaultOnly) {
-      if (obj.default) {
+      if ('default' in obj) {
         obj = obj.default;
       }
     }

test/fixtures/cjs/module-exports-null.js

@@ -0,0 +1 @@
+module.exports = null;

test/fixtures/esm/export-default-null.js

@@ -0,0 +1 @@
+export default null;

test/import.test.ts

@@ -83,5 +83,29 @@ describe('test/import.test.ts', () => {
       assert.equal(obj.foo, 'bar');
       assert.equal(obj.one, 1);
     });
+
+    it('should support module.exports = null', async () => {
+      assert.equal(await importModule(getFilepath('cjs/module-exports-null.js'), {
+        importDefaultOnly: true,
+      }), null);
+      assert.equal(await importModule(getFilepath('cjs/module-exports-null'), {
+        importDefaultOnly: true,
+      }), null);
+      assert.equal((await importModule(getFilepath('cjs/module-exports-null'), {
+        importDefaultOnly: false,
+      })).default, null);
+    });
+
+    it('should support export default null', async () => {
+      assert.equal(await importModule(getFilepath('esm/export-default-null.js'), {
+        importDefaultOnly: true,
+      }), null);
+      assert.equal(await importModule(getFilepath('esm/export-default-null'), {
+        importDefaultOnly: true,
+      }), null);
+      assert.equal((await importModule(getFilepath('esm/export-default-null.js'), {
+        importDefaultOnly: false,
+      })).default, null);
+    });
   });
 });