Tooltip HTML tags overlay Agile Backlog board items and other formatting issues due to broken avatar images

Summary

When there are broken avatar images for profiles (for example if you remove the images that were being pointed by avatar profiles or when not migrating the "data" folder from your jira-home), tickets that have the assignee with a broken avatar will appear with tooltips and tags over it, making the user struggling to see the ticket. (See attachment 1 and 2)
Also, the Assignee's avatar on the ticket view is replaced by the user's username, also causing formatting issues on the ticket. (See attachment 3).

Steps to reproduce

1. Delete the images from jira-home/data/avatars, or doesn't migrate the data folder when upgrading the instance
2. On an Agile Backlog board, select a project where there will be tickets with the assignee being one user that has a broken avatar image.

Expected Results

Set the default JIRA avatar to any user that have a broken avatar image (one that is pointing to a file that does not exist anymore), so that tickets appear normally and there are no formatting issues on the user's avatar image on the ticket view.

Actual Results

On the board, the ticket will appear with tooltips tags all over it, and on the ticket view, the assignee avatar image will be replaced with the user's username causing formatting errors.

Workaround

1. If you have upgrade or migrated your instance, remember to also migrate the data folder from your previous instance so attachments and avatars are also migrated.
2. If you have deleted the avatars images, set the default image for every user on the instance, making sure there is no user pointing its avatar image to a non existent image.