diff: Fix crash when remote object doesn't exist

Since we're saving nil in an interface rather than the implementation,
we can't compare to nil to check if the remote object exists or
not. Change the struct to save in the implementation.

test/cmd/diff.sh

@@ -26,6 +26,10 @@ run_kubectl_diff_tests() {
     create_and_use_new_namespace
     kube::log::status "Testing kubectl alpha diff"
 
+    # Test that it works when the live object doesn't exist
+    output_message=$(kubectl alpha diff LOCAL LIVE -f hack/testdata/pod.yaml)
+    kube::test::if_has_string "${output_message}" 'test-pod'
+
     kubectl apply -f hack/testdata/pod.yaml
 
     # Ensure that selfLink has been added, and shown in the diff