Update test to return null instead of throwing an error when there are no changes

- Modified the test case to expect getGitSummary to return null when there are no changes instead of throwing an error.
- Updated the mock implementation

Author: ryota-murakami

tests/index.test.js

@@ -78,16 +78,13 @@ describe('Git GPT Commit', () => {
       expect(gitGptCommit.getGitSummary).toHaveBeenCalled()
     })
 
-    it('throws an error when there are no changes', async () => {
+    it('returns null when there are no changes', async () => {
       // Temporarily modify the mock to simulate no changes
-      vi.mocked(gitGptCommit.getGitSummary).mockImplementationOnce(() => {
-        throw new Error('No changes to commit')
-      })
+      vi.mocked(gitGptCommit.getGitSummary).mockResolvedValueOnce(null)
 
       // Call getGitSummary with no changes
-      expect(async () => gitGptCommit.getGitSummary()).toThrow(
-        'No changes to commit',
-      )
+      const result = await gitGptCommit.getGitSummary()
+      expect(result).toBeNull()
     })
   })