How to get avatar for unassigned (question mark avatar) in JIRA

I am developing a plugin for JIRA. Plugin lists issues with details like assigned user, priority, etc. In my speed template file I have the following statement to get the avatar for the assigned user

... $ {avatarService.getAvatarAbsoluteURL ($ {user}, $ {user.getName ()}, $ Avatar.Size.SMALL)} ...

However, if no issue is assigned, the admin avatar is displayed. But when I open the issue to see the details. JIRA shows the assigned user as "Not assigned" and the quest tagged avatar displays correctly. Please help solve this problem.

Thanks in advance,

Shashi