[dashboard] Show userid in greeting if no different display name is set

There are two examples from our work:

1. Our organisation name contains a capitalisation and a character in the word -> ‘Bei.Spiel’.
   However, in the profile box under ‘Personal information’ in the settings, everything that is entered in the ‘Full name’ field is displayed correctly.

2. A person wants their name to be written in lower case -> ‘name’
   However, everything entered in the ‘Full name’ field is displayed correctly in the profile box under ‘Personal information’ in the settings.

p.s. Unfortunately, the automatically generated avatar icon with the initial character does not take lower case into account either. And I have just realised that there are several other places in the user interface where lower case is not represented.

I would like to see all names displayed.
And that the profile box also displays the initial lower case character.

We use two different instances, each with maintained versions of Nextcloud 28.

Start by reproducing the greeting, profile box, and avatar behavior described in the screenshots on a Nextcloud 28 instance, then trace the UI components that select and render user names. Done means the userid is shown when no different display name is set, while configured capitalization and lowercase initials are preserved across the reported interface locations.