fix show -json tests

The prior state recorded in the plans did not match the actual prior
state. Make the plans and state match depending on whether there was
existing state or not.

command/testdata/show-json/basic-create/output.json

@@ -53,18 +53,7 @@
             ]
         }
     },
-    "prior_state": {
-        "format_version": "0.1",
-        "values": {
-            "outputs": {
-                "test": {
-                    "sensitive": false,
-                    "value": "bar"
-                }
-            },
-            "root_module": {}
-        }
-    },
+    "prior_state": {},
     "resource_changes": [
         {
             "address": "test_instance.test[0]",