Only display merge commit SHA when it exists

What does this MR do?

There are several cases where a merged request can have no merge_commit_sha.

• When importer is used
• When fast-forward merge is used
• When branch is implicitly merged as another branch with same SHA has been merged.

In those cases it, fix the 'merged with` wording if merge commit is not present.

What are the relevant issue numbers?

#48889 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #48889 (closed)