fix: Allow null trait values

lib/src/core/model/trait.dart

@@ -5,6 +5,9 @@ import 'package:json_annotation/json_annotation.dart';
 import 'identity.dart';
 part 'trait.g.dart';
 
+// Default for copyWith's value, so that an explicit null can be told apart.
+const _unset = Object();
+
 @JsonSerializable()
 class Trait {
   @JsonKey(includeIfNull: false)
@@ -32,12 +35,12 @@ class Trait {
   Trait copyWith({
     int? id,
     String? key,
-    dynamic value,
+    dynamic value = _unset,
   }) =>
       Trait(
         id: id ?? this.id,
         key: key ?? this.key,
-        value: value ?? this.value,
+        value: identical(value, _unset) ? this.value : value,
       );
 
   static dynamic _fromJson(dynamic jsonValue) {
@@ -47,14 +50,19 @@ class Trait {
       } else {
         return jsonValue.toDouble(); // Treat as double if it's a decimal
       }
-    } else if (jsonValue is String || jsonValue is bool) {
+    } else if (jsonValue == null || jsonValue is String || jsonValue is bool) {
       return jsonValue;
     }
     throw ArgumentError('Invalid value type');
   }
 
   static dynamic _toJson(dynamic value) {
-    if (value is double || value is int || value is String || value is bool) {
+    // A null value deletes the trait in Flagsmith.
+    if (value == null ||
+        value is double ||
+        value is int ||
+        value is String ||
+        value is bool) {
       return value;
     }
     throw ArgumentError('Invalid value type');
@@ -89,12 +97,12 @@ class TraitWithIdentity {
   TraitWithIdentity copyWith({
     Identity? identity,
     String? key,
-    dynamic value,
+    dynamic value = _unset,
   }) =>
       TraitWithIdentity(
         identity: identity ?? this.identity,
         key: key ?? this.key,
-        value: value ?? this.value,
+        value: identical(value, _unset) ? this.value : value,
       );
 
   static dynamic _fromJson(dynamic jsonValue) {
@@ -104,14 +112,19 @@ class TraitWithIdentity {
       } else {
         return jsonValue.toDouble(); // Treat as double if it's a decimal
       }
-    } else if (jsonValue is String || jsonValue is bool) {
+    } else if (jsonValue == null || jsonValue is String || jsonValue is bool) {
       return jsonValue;
     }
     throw ArgumentError('Invalid value type');
   }
 
   static dynamic _toJson(dynamic value) {
-    if (value is double || value is int || value is String || value is bool) {
+    // A null value deletes the trait in Flagsmith.
+    if (value == null ||
+        value is double ||
+        value is int ||
+        value is String ||
+        value is bool) {
       return value;
     }
     throw ArgumentError('Invalid value type');

test/core/models/trait_test.dart

@@ -104,6 +104,45 @@ void main() {
       expect(mapped['trait_value'], 'value');
     });
   });
+  group('[Trait] - null value', () {
+    test('When trait value is null, then it is parsed as null', () {
+      final trait = Trait.fromJson({'trait_key': 'plan', 'trait_value': null});
+      expect(trait.value, isNull);
+    });
+
+    test('When trait value is null, then it is serialised as null', () {
+      final mapped = Trait(key: 'plan', value: null).toJson();
+      expect(mapped, {'trait_key': 'plan', 'trait_value': null});
+      expect(Trait.fromJson(jsonDecode(jsonEncode(mapped))).value, isNull);
+    });
+
+    test('When trait with identity value is null, then it round-trips', () {
+      final trait = TraitWithIdentity(
+          identity: Identity(identifier: 'user'), key: 'plan', value: null);
+      final mapped = trait.toJson();
+      expect(mapped['trait_value'], isNull);
+      expect(mapped.containsKey('trait_value'), isTrue);
+      expect(TraitWithIdentity.fromJson(mapped).value, isNull);
+    });
+
+    test('When copyWith sets value to null, then value is null', () {
+      final trait = Trait(key: 'plan', value: 'premium');
+      expect(trait.copyWith(value: null).value, isNull);
+      expect(trait.copyWith(key: 'tier').value, 'premium');
+    });
+
+    test(
+        'When trait with identity copyWith sets value to null, then value '
+        'is null', () {
+      final trait = TraitWithIdentity(
+          identity: Identity(identifier: 'user'),
+          key: 'plan',
+          value: 'premium');
+      expect(trait.copyWith(value: null).value, isNull);
+      expect(trait.copyWith(key: 'tier').value, 'premium');
+    });
+  });
+
   group('[Trait] - copyWith', () {
     test('as a developer want to use with new data', () {
       final trait = Trait.fromJson(decodedTraitStringValue);

test/fg/flagsmith_traits_test.dart

@@ -269,6 +269,24 @@ void main() {
       expect(result, isNotEmpty);
     });
 
+    test('When get flags with a null trait, then it is sent as null', () async {
+      var user = Identity(identifier: 'test_another_user');
+      fs = await setupClientAdapter(StorageType.inMemory);
+      setupEmptyAdapter(fs, cb: (config, adapter) {
+        adapter.onPost(fs.config.identitiesURI, (server) {
+          return server.reply(200, jsonDecode(identitiesResponseData));
+        }, data: {
+          'identifier': 'test_another_user',
+          'traits': [
+            {'trait_key': 'plan', 'trait_value': null},
+          ],
+        });
+      });
+      final result = await fs.getFeatureFlags(
+          user: user, traits: [Trait(key: 'plan', value: null)]);
+      expect(result, isNotEmpty);
+    });
+
     test('When get flags for transient identity request expected', () async {
       var user = Identity(identifier: 'test_another_user', transient: true);
       fs = await setupClientAdapter(StorageType.inMemory);