diff.json returns 404 for empty commits

Summary

Changes tab doesn't load when comparing with previous version which contains an empty commit.

Steps to reproduce

• create new branch from default branch
• create merge request from branch (without any commits)
• create empty commit on the branch
• in the system note for the push on the merge request page lick on Compare with previous version

Example

• Click Compare with previous version in gitlab-foss!21374 (comment 96756591)
• The changes tab doesn't load because https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/21374/diffs.json?diff_id=21822823&start_sha=7a9558a1521062d5f5c49a3bfda615afd60ba668 returns a 404 error page.

What is the current bug behavior?

error is shown and changes tab doesn't load

What is the expected correct behavior?

tab is shown without error