helper/schema: write "attr.#": "0" for empty maps

This fixes some perpetual diffs I saw in Atlas AccTests where an empty
map (`map[string]interface{}{}`) was being `d.Set` for "metadata_full".

Because the MapFieldWriter was not distinguishing between empty and nil,
this trigger the "map delete" logic and no count was written to the
state. This caused subsequent plans to improperly report a diff.

Here we redefine the map delete functionality to explicitly trigger only
on `nil`, so we catch the `.#` field for empty maps.

helper/schema/field_writer_map.go

@@ -141,7 +141,12 @@ func (w *MapFieldWriter) setMap(
 	v := reflect.ValueOf(value)
 	vs := make(map[string]interface{})
 
-	if value != nil {
+	if value == nil {
+		// The empty string here means the map is removed.
+		w.result[k] = ""
+		return nil
+	}
+
 	if v.Kind() != reflect.Map {
 		return fmt.Errorf("%s: must be a map", k)
 	}
@@ -152,13 +157,6 @@ func (w *MapFieldWriter) setMap(
 		mv := v.MapIndex(mk)
 		vs[mk.String()] = mv.Interface()
 	}
-	}
-
-	if len(vs) == 0 {
-		// The empty string here means the map is removed.
-		w.result[k] = ""
-		return nil
-	}
 
 	// Remove the pure key since we're setting the full map value
 	delete(w.result, k)

helper/schema/resource_data_test.go

@@ -2539,7 +2539,9 @@ func TestResourceDataState(t *testing.T) {
 			},
 
 			Result: &terraform.InstanceState{
-				Attributes: map[string]string{},
+				Attributes: map[string]string{
+					"tags.#": "0",
+				},
 			},
 		},