Properly filter dayData entries

This is needed when the "needs logs to advance" toggle is active

See https://github.com/wger-project/wger/issues/2164

test/routine/models/routine_test.dart

@@ -0,0 +1,42 @@
+/*
+ * This file is part of wger Workout Manager <https://github.com/wger-project>.
+ * Copyright (c) 2020 - 2026 wger Team
+ *
+ * wger Workout Manager is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+import 'package:flutter_test/flutter_test.dart';
+
+import '../../../test_data/routines.dart';
+
+void main() {
+  group('Routine model tests', () {
+    test('correctly filters out null days', () {
+      // Arrange
+      final routine = getTestRoutine();
+      routine.dayData = [
+        getTestRoutine().dayData[0],
+        getTestRoutine().dayData[0],
+        getTestRoutine().dayData[0],
+      ];
+      routine.dayData[0].date = DateTime(2026, 1, 1);
+      routine.dayData[1].date = DateTime(2026, 1, 2);
+      routine.dayData[2].date = DateTime(2026, 1, 3);
+
+      // Assert
+      expect(routine.dayDataCurrentIteration.length, equals(3));
+      expect(routine.dayDataCurrentIterationFiltered.length, equals(1));
+    });
+  });
+}