Fix EVP_PKEY_can_sign() handling of NULL from query_operation_name()

Fix EVP_PKEY_can_sign() handling of NULL from query_operation_name()

EVP_PKEY_can_sign() assumed query_operation_name(OSSL_OP_SIGNATURE)
always returns a non-NULL string. According to the documentation,
query_operation_name() may return NULL, in which case
EVP_KEYMGMT_get0_name() should be used as a fallback.

Fixes #27790

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28641)