I think we should allow --use-krb5-ccache="" to indicate usage of the default ccache...
This bug was referenced in samba master: 994e12e8f7a6b089342a32a6c3269048bfef1545 0ba9e5dacbb4e5bf94600e0a4a1cbd9f7a9c5d9e aff2932c420fd102c077063b8d1f66cdd8a777cb 2674df4cc0e124d74eb9d764c29a07c9c84b94d6