Resolve "Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits::Error: #<NoMethodError: undefined method `map' for nil:NilClass"

What does this MR do?

Handle null values in the st_commits column when doing the background migration.

Are there points in the code the reviewer needs to double check?

Don't think so! We already handle this case for diffs, but didn't for commits 🤦

Why was this MR needed?

We can't finish migrating with this bug.

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39181 (closed).