Server: it shouldn't be possible to create usersid's with spaces in it

[szaimen]

Steps to reproduce

create a userid with spaces in it like: "Max Mustermann"

Expected behaviour

there should be an error message, that this didn't work.

Actual behaviour

It just works

Server configuration

Operating system:
Ubuntu 18.03 LTS
Web server:
Apache
Database:
Pgsql
PHP version:
7.2
Nextcloud version: (see Nextcloud admin page)
17.0.2
Updated from an older Nextcloud/ownCloud or fresh install:
fresh
Where did you install Nextcloud from:
Nextcloud VM

While investigating possible testscenarios and userid's for https://github.com/nextcloud/vm/pull/1030, we experienced, that this is possible, but it shouldn't, because e.g. Federation wouldn't work. Also when logging in with that user, this happens:

[kesselb]

https://github.com/nextcloud/server/issues/18855

[enoch85]

Closing as duplicate.