return type nullability

[arcnmx]

Make sure gir produces valid code when turning on trust_return_value_nullability

TODO:

• [ ] audit signal argument types (this caused #30)
  • [ ] audit that signals.rs match the gir-generated prototypes
• [ ] audit everything manually beyond fns listed below (#29 should not have happened with trust_return_value_nullability=false but it did!)

Audit all changed function signatures:

• [ ] wp_core_get_properties
• [ ] wp_core_get_remote_host_name
• [ ] wp_core_get_remote_name
• [ ] wp_core_get_remote_properties
• [ ] wp_core_get_remote_user_name
• [ ] wp_core_get_remote_version
• [ ] wp_dbus_get_connection
• [ ] wp_dbus_get_instance
• [ ] wp_endpoint_get_media_class
• [ ] wp_endpoint_get_name
• [ ] wp_global_proxy_get_global_properties
• [ ] wp_metadata_new_iterator
• [ ] wp_node_new_ports_filtered_iterator_full
• [ ] wp_node_new_ports_iterator
• [ ] wp_object_get_core
• [ ] wp_object_manager_new_filtered_iterator_full
• [ ] wp_object_manager_new_iterator
• [ ] wp_pipewire_object_get_properties
• [ ] wp_pipewire_object_new_properties_iterator
• [ ] wp_properties_copy
• [ ] wp_properties_ensure_unique_owner
• [ ] wp_properties_new_iterator
• [ ] wp_properties_iterator_item_get_key
• [ ] wp_properties_iterator_item_get_value
• [ ] wp_properties_item_get_key
• [ ] wp_properties_item_get_value
• [ ] wp_session_item_get_properties
• [ ] wp_session_item_get_property
• [ ] wp_si_endpoint_get_registration_info
• [ ] wp_si_factory_construct
• [ ] wp_si_factory_get_name
• [ ] wp_si_link_get_in_item
• [ ] wp_si_link_get_out_item
• [ ] wp_si_link_get_registration_info
• [ ] wp_si_linkable_get_ports
• [ ] wp_spa_device_get_managed_object
• [ ] wp_spa_device_get_properties
• [ ] wp_spa_device_new_managed_object_iterator
• [ ] wp_spa_json_copy
• [ ] wp_spa_json_ensure_unique_owner
• [ ] wp_spa_json_get_data
• [ ] wp_spa_json_new_iterator
• [ ] wp_spa_json_parse_string
• [ ] wp_spa_json_builder_end
• [ ] wp_spa_json_parser_get_string
• [ ] wp_spa_pod_copy
• [ ] wp_spa_pod_ensure_unique_owner
• [ ] wp_spa_pod_filter
• [ ] wp_spa_pod_get_array_child
• [ ] wp_spa_pod_get_choice_child
• [ ] wp_spa_pod_new_iterator
• [ ] wp_spa_pod_builder_end
• [ ] wp_state_get_location
• [ ] wp_state_get_name
• [ ] wp_state_load
• [ ] wp_transition_get_source_object
• [ ] wp_spa_pod_get_spa_type