Fix notification email for image diff notes

What are the relevant issue numbers?

Fixes https://sentry.gitlab.net/gitlab/gitlabcom/issues/590898/. Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/53678.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Conforms to the code review guidelines