Fix MergeRequestsResolver

What does this MR do?

#208431 (closed)

Follow-up from discussion.

In Resolvers::MergeRequestsResolver if the project was not found, BatchLoader object can be nil and project.present? would still return true.

With these changes, we use sync in the BatchLoader object and evaluate if the result is nil, similarly to what is done in Resolvers::IssuesResolver

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #208431 (closed)