fix: Prevent "null" identifier collisions from malformed drink JSON

lib/models/drink.dart

@@ -27,8 +27,8 @@ class Producer {
     }
 
     return Producer(
-      id: json['id'].toString(),
-      name: json['name'].toString(),
+      id: (json['id'] as String?) ?? '',
+      name: (json['name'] as String?) ?? '',
       location: (json['location'] ?? '').toString(),
       yearFounded: yearFounded,
       notes: json['notes']?.toString(),
@@ -139,8 +139,8 @@ class Product {
     }
 
     return Product(
-      id: json['id'].toString(),
-      name: json['name'].toString(),
+      id: (json['id'] as String?) ?? '',
+      name: (json['name'] as String?) ?? '',
       category: (json['category'] ?? 'beer').toString(),
       style: json['style']?.toString(),
       dispense: (json['dispense'] ?? 'cask').toString(),

lib/services/beer_api_service.dart

@@ -109,7 +109,9 @@ class BeerApiService {
 
     for (final producerJson in producers) {
       final producer = Producer.fromJson(producerJson as Map<String, dynamic>);
+      if (producer.id.isEmpty) continue;
       for (final product in producer.products) {
+        if (product.id.isEmpty) continue;
         drinks.add(Drink(
           product: product,
           producer: producer,

test/beer_api_service_test.dart

@@ -241,6 +241,104 @@ void main() {
 
         expect(drinks.first.festivalId, 'my-festival-id');
       });
+
+      test('skips products with missing ids', () async {
+        service = BeerApiService(client: mockClient);
+
+        const festival = Festival(
+          id: 'cbf2025',
+          name: 'Test Festival',
+          dataBaseUrl: 'https://example.com',
+          availableBeverageTypes: ['beer'],
+        );
+
+        final responseBody = json.encode({
+          'producers': [
+            {
+              'id': 'brewery-1',
+              'name': 'Test Brewery',
+              'location': 'Cambridge',
+              'products': [
+                {
+                  'id': null,
+                  'name': 'No Id Beer',
+                  'category': 'beer',
+                  'dispense': 'cask',
+                  'abv': '4.0',
+                },
+                {
+                  'id': 'drink-2',
+                  'name': 'Valid Beer',
+                  'category': 'beer',
+                  'dispense': 'cask',
+                  'abv': '4.5',
+                },
+              ],
+            },
+          ],
+        });
+
+        when(mockClient.get(Uri.parse('https://example.com/beer.json')))
+            .thenAnswer((_) async => http.Response(responseBody, 200));
+
+        final drinks = await service.fetchDrinks(festival, 'beer');
+
+        expect(drinks.length, 1);
+        expect(drinks.first.id, 'drink-2');
+      });
+
+      test('skips producers with missing ids', () async {
+        service = BeerApiService(client: mockClient);
+
+        const festival = Festival(
+          id: 'cbf2025',
+          name: 'Test Festival',
+          dataBaseUrl: 'https://example.com',
+          availableBeverageTypes: ['beer'],
+        );
+
+        final responseBody = json.encode({
+          'producers': [
+            {
+              'id': null,
+              'name': 'Missing Id Brewery',
+              'location': 'Cambridge',
+              'products': [
+                {
+                  'id': 'drink-ignored',
+                  'name': 'Ignored Beer',
+                  'category': 'beer',
+                  'dispense': 'cask',
+                  'abv': '4.0',
+                },
+              ],
+            },
+            {
+              'id': 'brewery-2',
+              'name': 'Valid Brewery',
+              'location': 'Cambridge',
+              'products': [
+                {
+                  'id': 'drink-2',
+                  'name': 'Valid Beer',
+                  'category': 'beer',
+                  'dispense': 'cask',
+                  'abv': '4.5',
+                },
+              ],
+            },
+          ],
+        });
+
+        when(mockClient.get(Uri.parse('https://example.com/beer.json')))
+            .thenAnswer((_) async => http.Response(responseBody, 200));
+
+        final drinks = await service.fetchDrinks(festival, 'beer');
+
+        expect(drinks.length, 1);
+        expect(drinks.first.id, 'drink-2');
+        expect(drinks.first.producer.id, 'brewery-2');
+      });
     });
 
     group('fetchAllDrinks', () {

test/models_test.dart

@@ -52,6 +52,19 @@ void main() {
       expect(product.allergens, isEmpty);
     });
 
+    test('fromJson maps missing id and name to empty strings', () {
+      final product = Product.fromJson({
+        'id': null,
+        'name': null,
+        'category': 'beer',
+        'dispense': 'cask',
+        'abv': '4.0',
+      });
+
+      expect(product.id, '');
+      expect(product.name, '');
+    });
+
     test('availabilityStatus returns correct values', () {
       expect(
         Product.fromJson({
@@ -722,6 +735,18 @@ void main() {
       expect(producer.location, '');
     });
 
+    test('fromJson maps missing id and name to empty strings', () {
+      final producer = Producer.fromJson({
+        'id': null,
+        'name': null,
+        'location': 'Somewhere',
+        'products': [],
+      });
+
+      expect(producer.id, '');
+      expect(producer.name, '');
+    });
+
     group('toJson', () {
       test('converts producer to JSON correctly', () {
         const product = Product(