Avoid returning without written ctx when posting PR (#31843)

[GiteaBot]

Backport #31843 by @wolfogre

Fix #31625.

If pull_service.NewPullRequest return an error which misses each if check, CompareAndPullRequestPost will return immediately, since it doesn't write the HTTP response, a 200 response with empty body will be sent to clients.

	if err := pull_service.NewPullRequest(ctx, repo, pullIssue, labelIDs, attachments, pullRequest, assigneeIDs); err != nil {
		if repo_model.IsErrUserDoesNotHaveAccessToRepo(err) {
			ctx.Error(http.StatusBadRequest, "UserDoesNotHaveAccessToRepo", err.Error())
		} else if git.IsErrPushRejected(err) {
			// ...
			ctx.JSONError(flashError)
		} else if errors.Is(err, user_model.ErrBlockedUser) {
			// ...
			ctx.JSONError(flashError)
		} else if errors.Is(err, issues_model.ErrMustCollaborator) {
			// ...
			ctx.JSONError(flashError)
		}
		return
	}

Not sure what kind of error can cause it to happen, so this PR just expose it. And we can fix it when users report that creating PRs failed with error responses.

It's all my guess since I cannot reproduce the problem, but even if it's not related, the code here needs to be improved.