User initial don't display properly in girder4

[naglepuff]

During the calculation of user initials in LoginMenue.vue, there is no fallback if first_name and last_name are '', null or undefined. This doesn't break anything, but we should have placeholder values for this case, and trying to squeeze undefinedundfined into the user icon looks bad.

[waxlamp]

Not to get too philosophical, but if undefinedundefined makes it to the end user, I consider that a broken part of the client.

[JackWilb]

Closed via #204