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')

#230
!280