Fix unexpected exception by failure of finding an actual head pipeline

What does this MR do?

As https://gitlab.com/gitlab-org/gitlab-ce/issues/56113 indicates, sometimes MergeRequest#find_actual_head_pipeline returns nil as failed to find an actual pipeline. This could happen by a race condition that a pipeline corresponding to the diff_head_sha has not been created yet. In this case, we should not raise an exception, nor change the head pipeline to nil. We simply update head pipeline only if a new pipeline is found.

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/56113

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [-] Documentation created/updated
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer