Fix nuxeo-user-avatar

How to reproduce

• as an Administrator create 3 users
• the 1st only with username: Bart (rest of the not required fields in blank)
• the 2nd only with username: Beer + first name: Homer
• the 3rd only with username: Saxophone + last name: Simpson
• go to the user group management page
• look to the nuxeo-user-avatar of the new created users in the list

Results:

• the one only with username doesn't have an avatar (error)
• the one only username + first name has HN Homer null (error)
• the one only username + last name has NS null Simpson (error)

The nuxeo-user-avatar should always appear and it should only have the filled fields in account.