Fix missing sha default in merge trains API

What does this MR do and why?

MergeRequests::MergeService requires merge_request.merge_params['sha'] to be present and up-to-date in order to merge. Otherwise it fails with an error stating that the branch is out of date.

See #409339 (closed)

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Before	After

How to set up and validate locally

1. Create a project with merge trains enabled
2. Open a merge request
3. With GDK_TOKEN, PROJECT_ID and MR_IID set in your environment, run:

   curl --request POST --header "PRIVATE-TOKEN: $GDK_TOKEN" \
     "https://gdk.test:3000/api/v4/projects/$PROJECT_ID/merge_trains/merge_requests/$MR_IID"

4. On the MR page, observe that a merge train starts and the merge request gets merged.

MR acceptance checklist

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