fix(github): guard pushFiles against empty diff from diffCommitTree

GitHub's POST /git/trees rejects an empty tree array with 422
"Invalid tree info". While prepareCommit already prevents no-op
commits from reaching pushFiles, add a defensive early return so
the failure mode is a clear log message instead of a swallowed 422.

Made-with: Cursor

Author: jiridanek

lib/modules/platform/github/index.spec.ts

@@ -5090,7 +5090,14 @@ describe('modules/platform/github/index', () => {
       git.getCommitTreeSha.mockResolvedValue(
         '0000000000000000000000000000000000000000' as LongCommitSha,
       );
-      git.diffCommitTree.mockResolvedValue([]);
+      git.diffCommitTree.mockResolvedValue([
+        {
+          path: 'foo.bar',
+          mode: '100644',
+          type: 'blob',
+          sha: 'abc0000000000000000000000000000000000000',
+        },
+      ]);
     });
 
     it('returns null if pre-commit phase has failed', async () => {
@@ -5128,6 +5135,21 @@ describe('modules/platform/github/index', () => {
       expect(res).toBeNull();
     });
 
+    it('returns null when diff is empty', async () => {
+      const scope = httpMock.scope(githubApiHost);
+      initRepoMock(scope, 'some/repo');
+      await github.initRepo({ repository: 'some/repo' });
+      git.diffCommitTree.mockResolvedValueOnce([]);
+
+      const res = await github.commitFiles({
+        branchName: 'foo/bar',
+        files: [{ type: 'addition', path: 'foo.bar', contents: 'foobar' }],
+        message: 'Foobar',
+      });
+
+      expect(res).toBeNull();
+    });
+
     it('commits and returns SHA string', async () => {
       const scope = httpMock.scope(githubApiHost);
 

lib/modules/platform/github/index.ts

@@ -2171,6 +2171,14 @@ async function pushFiles(
     const baseTreeSha = await getCommitTreeSha(parentCommitSha);
     const treeItems = await diffCommitTree(parentCommitSha, commitSha);
 
+    if (treeItems.length === 0) {
+      logger.debug(
+        { branchName },
+        'Platform-native commit: no changed files between commits',
+      );
+      return null;
+    }
+
     const treeRes = await githubApi.postJson<{ sha: string }>(
       `/repos/${config.repository}/git/trees`,
       { body: { base_tree: baseTreeSha, tree: treeItems } },