Handle password=0 for z_HTTP_header_add_basic_auth
Handle password=0 for z_HTTP_header_add_basic_auth

Avoid 0-ptr reference for z_HTTP_header_add_basic_auth . If NULL
password is passed that is treated as treated as empty string.

nla/yaz,dcrossleyau/yaz,nla/yaz,dcrossleyau/yaz,nla/yaz,nla/yaz,dcrossleyau/yaz