DP: add NULL check to be_ptask_{enable|disable}
    
    Currently the files and the proxy provider do not provide a check online
    method (DPM_CHECK_ONLINE). The files provider because it can never go
    offline. The proxy provider because there is no generic way to check
    since the nature of the actual provider is unknown.
    
    Since the method is missing check_if_online() jumps into the error
    handling block were we try to reset the offline state
    unconditionally. If there is no check_if_online_ptask, which never
    exists for the files provider and will not be available in the proxy
    provider as long as the backend is online, be_ptask_{enable|disable}
    will be called with a NULL pointer.
    
    Related to https://pagure.io/SSSD/sssd/issue/4014
    
    Reviewed-by: Tomas Halman <thalman@redhat.com>
    Reviewed-by: Alexey Tikhonov <atikhono@redhat.com>