pam_sss: Fix checking of empty string cert_user
    
    src/sss_client/pam_sss.c: In function ‘eval_response’:
    src/sss_client/pam_sss.c:998:64: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
                     if (type == SSS_PAM_CERT_INFO && pi->cert_user == '\0') {
                                                                    ^~
    src/sss_client/pam_sss.c:998:50: note: did you mean to dereference the pointer?
                     if (type == SSS_PAM_CERT_INFO && pi->cert_user == '\0') {
                                                      ^
    src/sss_client/pam_sss.c:1010:42: error: comparison between pointer and zero character constant [-Werror=pointer-compare]
                             && pi->cert_user != '\0') {
                                              ^~
    src/sss_client/pam_sss.c:1010:28: note: did you mean to dereference the pointer?
                             && pi->cert_user != '\0') {
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>