Translation for 'show only nicknames' is incorrect

One-sentence description

If a user selects 'only initials' as their display name this displays their nickname. When editing a user in admin selecting 'only initials' and not filling out the nickname produces an error.

Current behaviour / Reproducing the bug 1

1. Select 'initials only' as display name in admin
2. Delete the nickname
3. Try to save the user
4. This produces the error that a nickname should be entered

Current behaviour / Reproducing the bug 2

1. Select 'initials only' as display name in admin
2. Go to profile page
3. Only your nickname is displayed

Expected behaviour

1. Select 'initials only' as display name in admin
2. Delete nickname but enter something for the initials
3. Try to save the user
4. This should work

Expected behaviour 2

1. Select 'initials only' as display name
2. Go to profile page
3. Only initials should be displayed