The fake client Update() should not store omit empty fields

[berlin-ab]

The update deserializes the json from the old object into the new object, but if the old object didn't have the json keys to override the new object, the new object keeps its values:

The relevant code is here:

https://github.com/kubernetes-sigs/controller-runtime/blob/main/pkg/client/fake/client.go#L1025

See #2476 for a test case that points out the bug.

[berlin-ab]

Fake client [It] should not change the status of typed objects that have a status subresource on update with omitempty fields
/Users/aberlin/workspace/controller-runtime/pkg/client/fake/client_test.go:1430

  [FAILED] Expected
      <v1.NodeStatus>: {
          Capacity: nil,
          Allocatable: nil,
          Phase: "",
          Conditions: nil,
          Addresses: nil,
          DaemonEndpoints: {
              KubeletEndpoint: {Port: 0},
          },
          NodeInfo: {
              MachineID: "",
              SystemUUID: "",
              BootID: "",
              KernelVersion: "",
              OSImage: "",
              ContainerRuntimeVersion: "",
              KubeletVersion: "",
              KubeProxyVersion: "",
              OperatingSystem: "",
              Architecture: "",
          },
          Images: nil,
          VolumesInUse: nil,
          VolumesAttached: nil,
          Config: {Assigned: nil, Active: nil, LastKnownGood: nil, Error: "some string"},
      }
  to be equivalent to
      <v1.NodeStatus>: {
          Capacity: nil,
          Allocatable: nil,
          Phase: "",
          Conditions: nil,
          Addresses: nil,
          DaemonEndpoints: {
              KubeletEndpoint: {Port: 0},
          },
          NodeInfo: {
              MachineID: "",
              SystemUUID: "",
              BootID: "",
              KernelVersion: "",
              OSImage: "",
              ContainerRuntimeVersion: "",
              KubeletVersion: "",
              KubeProxyVersion: "",
              OperatingSystem: "",
              Architecture: "",
          },
          Images: nil,
          VolumesInUse: nil,
          VolumesAttached: nil,
          Config: nil,
      }
      ```

[troy0820]

/kind support bug

[alvaroaleman]

Fixed in #2484