kdumpctl: fix check_config error when kdump.conf is empty
    
    Resolves: bz1975632
    Upstream: Fedora
    Conflict: None
    
    commit 6137956f79e8dc79bc1048efbe9bccbd66ac61e9
    Author: Kairui Song <kasong@redhat.com>
    Date:   Wed Apr 14 13:41:19 2021 +0800
    
        kdumpctl: fix check_config error when kdump.conf is empty
    
        Kdump scirpt already have default values for core_collector, path in
        many other place. Empty kdump.conf still works. Fix this corner case and
        fix the error message.
    
        Signed-off-by: Kairui Song <kasong@redhat.com>
        Acked-by: Pingfan Liu <piliu@redhat.com>
    
    Signed-off-by: Tao Liu <ltao@redhat.com>