Fix RSA OAEP failure when given empty label with OpenSSL 3

[dagood]

Found this while running x/crypto internal/wycheproof tests using the OpenSSL backend.

• The bug is already reported to OpenSSL:
  Impossible to clear the label with EVP_PKEY_CTX_set0_rsa_oaep_label() openssl/openssl#21288.

The doc is pretty clear that what we were doing should work:

EVP_PKEY_CTX_set0_rsa_oaep_label() sets the RSA OAEP label to binary data label and its length in bytes to len. If label is NULL or len is 0, the label is cleared.

Also fixes the error message to match the function name.