fix(prepare): skip PR sync when there are zero updates and no changes to push

When no packages need updating, the release branch is identical to the default
branch. Previously we still called syncPullRequest, which attempted to create a
PR and hit a GitHub 422 ("No commits between main and release/next"). Now we
bail out early with a clear log message and return null instead.

Author: luxass

src/workflows/prepare.ts

@@ -253,6 +253,18 @@ export async function prepareWorkflow(options: NormalizedReleaseScriptsOptions):
   }
 
   if (!hasChangesToPush.value) {
+    // When there are no updates at all, the release branch is identical to the
+    // default branch.  Attempting to create/update a PR would fail with a 422
+    // ("No commits between main and <release-branch>"), so bail out early.
+    if (allUpdates.length === 0) {
+      logger.info("No changes to commit and no packages to release. Nothing to do.");
+      const checkoutResult = await checkoutBranch(options.branch.default, options.workspaceRoot);
+      if (!checkoutResult.ok) {
+        exitWithError(`Failed to checkout branch: ${options.branch.default}`, undefined, checkoutResult.error);
+      }
+      return null;
+    }
+
     const prResult = await syncPullRequest({
       github: options.githubClient,
       releaseBranch: options.branch.release,