agent-manager: return all secrets if we don't need to ask agents

When the connection already has all needed secrets, _con_get_try_complete_early() returns existing_secrets without asking to agents; existing_secrets is initialized with system secrets.

In NMSettingsConnection, get_secrets_done_cb() strips all secrets from the connection and then merges the secrets returned by nm_agent_manager_get_secrets(). Since nm_agent_manager_get_secrets() returns only system secrets, the result is that agent owned secrets are not asked to agents and are removed from the connection.

Fix this by returning all secrets from the agent manager.

https://gitlab.gnome.org/GNOME/network-manager-applet/-/issues/157