pf resources show diff for empty map

re pulumi/pulumi-aws#4080

I've noticed different behavior between sdkv2 and pf resources.
Sometimes if you do not provide a value for a map property it will write
an empty value to the outputs (i.e. `{ "tagsAll": {} }`).

When diff is called, the `olds` show the empty object value and the news
show no entry at all. sdkv2 resources handle this and do not display any
diff whereas pf resources show a diff.

pf/tests/provider_diff_test.go

@@ -195,3 +195,39 @@ func TestDiffVersionUpgrade(t *testing.T) {
         }`
 	testutils.Replay(t, server, testCase)
 }
+
+// Test shows that an empty object shows a diff if it is not provided
+func TestEmptyObjectDiff(t *testing.T) {
+	server := newProviderServer(t, testprovider.SyntheticTestBridgeProvider())
+	testCase := `
+        {
+          "method": "/pulumirpc.ResourceProvider/Diff",
+          "request": {
+            "id": "0",
+            "urn": "urn:pulumi:test-stack::basicprogram::testbridge:index/testres:Testres::testres1",
+						"olds": {
+							"id": "none",
+							"statedir": "res-461023c",
+							"requiredInputString": "abc",
+							"requiredInputStringCopy": "abc",
+							"optionalInputStringMap": {}
+						},
+						"news": {
+							"statedir": "res-461023c",
+							"requiredInputString": "abc"
+						},
+						"oldInputs": {
+							"statedir": "res-461023c",
+							"requiredInputString": "abc"
+						}
+          },
+					"response": {
+						"changes": "DIFF_SOME",
+						"diffs": [
+						"optionalInputStringMap"
+						]
+					}
+        }
+        `
+	testutils.Replay(t, server, testCase)
+}

pkg/tfbridge/tests/provider_test.go

@@ -70,6 +70,55 @@ func TestWithNewTestProvider(t *testing.T) {
 	`)
 }
 
+// An empty object does not show a diff if it is not provided
+func TestEmptyMapNoDiff(t *testing.T) {
+	ctx := context.Background()
+	p := newTestProvider(ctx, tfbridge.ProviderInfo{
+		P: shimv2.NewProvider(&schema.Provider{
+			Schema: map[string]*schema.Schema{},
+			ResourcesMap: map[string]*schema.Resource{
+				"example_resource": {
+					Schema: map[string]*schema.Schema{
+						"optional_input_string_map": {
+							Type: schema.TypeMap,
+							Elem: &schema.Schema{
+								Type: schema.TypeString,
+							},
+							Optional: true,
+						},
+					},
+				},
+			},
+		}),
+		Name:           "testprov",
+		ResourcePrefix: "example",
+		Resources: map[string]*tfbridge.ResourceInfo{
+			"example_resource": {Tok: "testprov:index:ExampleResource"},
+		},
+	}, newTestProviderOptions{})
+
+	replay.Replay(t, p, `
+	{
+		"method": "/pulumirpc.ResourceProvider/Diff",
+		"request": {
+			"id": "0",
+			"urn": "urn:pulumi:dev::teststack::testprov:index:ExampleResource::exres",
+			"olds": {
+				"optionalInputStringMap": {}
+			},
+			"news": {
+			},
+			"oldInputs": {
+			}
+		},
+		"response": {
+			"changes": "DIFF_NONE",
+			"hasDetailedDiff": true
+		}
+	}
+	`)
+}
+
 // TestRegress1932 tests that we can have a list with different types (string & unknown)
 func TestRegress1932(t *testing.T) {
 	ctx := context.Background()