[crypto] Fix debug name for empty certificate chain validators

[crypto] Fix debug name for empty certificate chain validators

An attempt to use a validator for an empty certificate chain will
correctly fail the overall validation with the "empty certificate
chain" error propagated from x509_auto_append().

In a debug build, the call to validator_name() will attempt to call
x509_name() on a non-existent certificate, resulting in garbage in the
debug message.

Fix by checking for the special case of an empty certificate chain.

This issue does not affect non-debug builds, since validator_name() is
(as per its description) called only for debug messages.

Signed-off-by: Michael Brown <mcb30@ipxe.org>