groups#show: display `@members` count, not `@group.members`.

The old was removed in this commit:
https://github.com/fiedl/wingolfsplattform/commit/14ffed6cd829ce089efefe
af8eb796b400ef3b54

Trello: https://trello.com/c/RPlYvbcv/645-gruppen-seite-schneller-machen
groups#show: display the number of members for large groups.
issue fixing: make specs green again.

In cases where the objects were `nil` or didn’t respond to the methods,
errors have been raised. This is fixed now.