Reject a certificate with an empty NSS nickname
    
    It technically wasn't NULL initially, it was "", so wasn't being
    caught in the NULL check. The value stored was "" which was treated
    as NULL when re-read so the request would eventually lead to a
    SEGFAULT by doing a strcmp.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1995681
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>