rsa: Accept NULL OAEP label for backward compatibility

rsa: Accept NULL OAEP label for backward compatibility

According to the manual page, EVP_PKEY_CTX_set0_rsa_oaep_label()
should accept NULL as the label argument, though the function
currently rejects it while setting the corresponding octet string
parameter with OSSL_PARAM_construct_octet_string, which expects
non-NULL input.  This adds a workaround to the caller for backward
compatibility.

Signed-off-by: Daiki Ueno <dueno@redhat.com>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22397)