KCM: Allow representing ccaches with a NULL principal
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3873
    
    We need to make it possible to create an internal ccache representation
    without passing in a principal. The principal is only assigned to the
    ccache with krb5_cc_initialize(), but some programs like openssh use the
    following sequence of calls:
        krb5_cc_new_unique
        krb5_cc_switch
        krb5_cc_initialize
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>