Editor: should not be able to diff if the resource in the editor doesn't exist on the cluster

[adietish]

Steps:

1. ASSERT: make sure that there's no pod busybox on the cluster
2. EXEC: open an editor with the following content:

apiVersion: v1
kind: Pod
metadata:
  name: busybox
spec:
  securityContext:
    runAsUser: 1000
    runAsGroup: 3000
    fsGroup: 2000
  containers:
    - name: busybox
      image: busybox
      ports:
        - containerPort: 8080

3. EXEC: hit "Diff" in the editor toolbar

Result: The diff dialog opens up with the remote portion empty.

Expected result: I should not be able to open the diff dialog.