cephadm-adopt: fix mgr placement hosts task

When no `[mgrs]` group is defined in the inventory, mgr daemon are
implicitly collocated with monitors.
This task currently relies on the length of the mgr group in order to
tell cephadm to deploy mgr daemons.
If there's no `[mgrs]` group defined in the inventory, it will ask
cephadm to deploy 0 mgr daemon which doesn't make sense and will throw
an error.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1970313



Signed-off-by: Guillaume Abrioux <gabrioux@redhat.com>