fix(dynamite_runtime): contentstring serialize null values

packages/dynamite/dynamite_runtime/lib/src/built_value/content_string_serializer.dart

@@ -21,6 +21,10 @@ class ContentStringPlugin implements SerializerPlugin {
       return object;
     }
 
+    if (object == null) {
+      return object;
+    }
+
     if (object is! Map<String, dynamic>) {
       throw StateError('ContentStringPlugin can only be applied to Map<String, dynamic>. '
           'Please ensure the StandardJsonPlugin is applied and run before.');
@@ -37,6 +41,10 @@ class ContentStringPlugin implements SerializerPlugin {
       return object;
     }
 
+    if (object == null) {
+      return object;
+    }
+
     if (object is! String) {
       throw StateError('The serialized ContentString must be of type String. '
           'Please ensure the StandardJsonPlugin is applied and run before.');

packages/dynamite/dynamite_runtime/test/content_string_test.dart

@@ -333,4 +333,18 @@ void main() {
       );
     });
   });
+
+  group('Serialize null value', () {
+    const data = null;
+    const serialized = null;
+    const specifiedType = FullType(ContentString, [FullType(bool)]);
+
+    test('can be serialized', () {
+      expect(serializers.serialize(data, specifiedType: specifiedType), serialized);
+    });
+
+    test('can be deserialized', () {
+      expect(serializers.deserialize(serialized, specifiedType: specifiedType), data);
+    });
+  });
 }