The Redfish Chassis schema (v1.27.0) defines the NetworkAdapters property
as an optional link to a collection of network adapters physically located
in the chassis. It is perfectly valid for this property to be omitted when
no adapters are present.

However, the current implementation of (*Chassis).NetworkAdapters() calls
ListReferencedNetworkAdapter even when the link is empty, potentially
resulting in 404 errors or misleading failures on otherwise valid systems.

This patch introduces an early return when the networkAdapters link is
not defined, aligning the behavior with other optional properties such as
Power(), Thermal(), and PCIeSlots().

This is particularly relevant for minimal chassis configurations like
backplanes or storage components (e.g., ChassisType: "Component" or "Module"),
which often appear in the Chassis collection but do not expose network
hardware.

Reference: https://redfish.dmtf.org/schemas/v1/Chassis.v1_27_0.json