FIX Group->canEdit() correct non-admin checks (fixes #8250)

Due to changed return value of DataObject::get(),
the (negated) check always returned false.
This wasn't noticed in 3.0 because Group->canEdit() is rarely
enforced, but does become noticeable in 3.1 where GridField
checks those object-level permissions.

Thanks to @purplespider for reporting!