s3:libsmb: explicitly use the default krb5 ccache in cli_session_creds_init() without...

s3:libsmb: explicitly use the default krb5 ccache in cli_session_creds_init() without a password

This happened implicitly as the gse_krb5 module always used the default
krb5 ccache, but that will change soon.

If kerberos is requested without a fallback to ntlm AND
the caller doesn't provide a password we'll use the
default ccache. This will keep our the following tests
happy once the gse_krb5 module changes the behavior:

 samba3.blackbox.krbsmbspool
 samba3.blackbox.smbget

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>