settings: fix updating agent-owned VPN secrets
property_to_dbus() returns NULL when called with NM_CONNECTION_SERIALIZE_WITH_SECRETS_AGENT_OWNED and the property is not an agent-owned secrets. The function doesn't handle VPN secrets correctly, since they are all stored as a hash in the vpn.secrets property and the flag for each of them is a matching '*-flags' key in the vpn.data property. VPN secrets must be handled differently; do it in the VPN setting to_dbus_fcn() function.
Fixes: 71928a3e ('settings: avoid cloning the connection to maintain agent-owned secrets')