Refactor node_has_curve25519_onion_key() to use node_get_curve25519_onion_key()
In #23577 (moved), we will create node_get_curve25519_onion_key(). We should use it to implement node_has_curve25519_onion_key(), so they give consistent results.
In #23577 (moved), we will create node_get_curve25519_onion_key(). We should use it to implement node_has_curve25519_onion_key(), so they give consistent results.