Make sure test has a valid ResourceState

Empty resources are now pruned more aggressively, so make sure there is
a valid ResourceState in the test ModuleState.

state/cache_test.go

@@ -71,7 +71,11 @@ func TestCacheState_RefreshState(t *testing.T) {
 				&terraform.ModuleState{
 					Path: terraform.RootModulePath,
 					Resources: map[string]*terraform.ResourceState{
-						"foo.foo": &terraform.ResourceState{},
+						"foo.foo": &terraform.ResourceState{
+							Primary: &terraform.InstanceState{
+								ID: "ID",
+							},
+						},
 					},
 				},
 			},