Fix pretty-printing empty serial number in ossl_serial_number_print()

Fix pretty-printing empty serial number in ossl_serial_number_print()

Fix a crash when the ASN1_INTEGER has empty content. While it is
illegal, this is the initial state of the serialNumber field when an
X509 object is allocated by X509_new(). X509_print*() should be able to
process an incomplete X509 object too.

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26557)