Disabling Gravatar causes epics to display Something went wrong while fetching child epics banner if issue assignee does not have an avatar in 12.5 through 12.7

Summary

Disabling Gravatar causes epics to display a red banner with Something went wrong while fetching child epics. when an issue with an assignee that does not have an avatar is added to the epic. No errors are thrown in the GitLab logs or in the browser console. Enabling Gravatar or adding an avatar to the user resolves the issue. This bug exists in 12.5-12.7 but is no longer present in 12.8

Steps to reproduce

1. Create an epic
2. Create an issue, set an assignee, and add it to the epic
3. Disable Gravatar and ensure that the assigned user does not have an avatar
4. Navigate to the Epic and see Something went wrong while fetching child epics. banner

What is the current bug behavior?

Disabling Gravatar causes epics to display a red banner with Something went wrong while fetching child epics. when an issue with an assignee that does not have an avatar is added to the epic. No errors are thrown in the GitLab logs or in the browser console. Enabling Gravatar or adding an avatar to the user resolves the issue. This bug exists in 12.5-12.7 but is no longer present in 12.8

What is the expected correct behavior?

An assignee without an avatar should not show the Something went wrong while fetching child epics. banner.

Possible fixes

Enable Gravatar or add an avatar for the assigned user.

This issue only exists in 12.5, 12.6, and 12.7.

Customer ticket: federal instance 65

/cc @kevinchasse