allow flutterManifest to handle empty lists, add unit tests (#81324)

packages/flutter_tools/lib/src/flutter_manifest.dart

@@ -495,22 +495,41 @@ void _validateFlutter(YamlMap? yaml, List<String> errors) {
         }
         break;
       case 'assets':
-        if (yamlValue is! YamlList || yamlValue[0] is! String) {
+        if (yamlValue is! YamlList) {
+
           errors.add('Expected "$yamlKey" to be a list, but got $yamlValue (${yamlValue.runtimeType}).');
+        } else if (yamlValue.isEmpty) {
+          break;
+        } else if (yamlValue[0] is! String) {
+          errors.add(
+            'Expected "$yamlKey" to be a list of strings, but the first element is $yamlValue (${yamlValue.runtimeType}).',
+          );
         }
         break;
       case 'fonts':
-        if (yamlValue is! YamlList || yamlValue[0] is! YamlMap) {
+        if (yamlValue is! YamlList) {
           errors.add('Expected "$yamlKey" to be a list, but got $yamlValue (${yamlValue.runtimeType}).');
+        } else if (yamlValue.isEmpty) {
+          break;
+        } else if (yamlValue.first is! YamlMap) {
+          errors.add(
+            'Expected "$yamlKey" to contain maps, but the first element is $yamlValue (${yamlValue.runtimeType}).',
+          );
         } else {
           _validateFonts(yamlValue, errors);
         }
         break;
       case 'licenses':
-        if (yamlValue is YamlList) {
-          _validateListType<String>(yamlValue, errors, '"$yamlKey"', 'files');
-        } else {
+        if (yamlValue is! YamlList) {
           errors.add('Expected "$yamlKey" to be a list of files, but got $yamlValue (${yamlValue.runtimeType})');
+        } else if (yamlValue.isEmpty) {
+          break;
+        } else if (yamlValue.first is! String) {
+          errors.add(
+            'Expected "$yamlKey" to contain strings, but the first element is $yamlValue (${yamlValue.runtimeType}).',
+          );
+        } else {
+          _validateListType<String>(yamlValue, errors, '"$yamlKey"', 'files');
         }
         break;
       case 'module':

packages/flutter_tools/test/general.shard/flutter_manifest_test.dart

@@ -767,6 +767,44 @@ flutter:
       contains('Expected "fonts" to either be null or a list.'));
   });
 
+  testWithoutContext('FlutterManifest ignores empty list of fonts', () {
+    const String manifest = '''
+name: test
+dependencies:
+  flutter:
+    sdk: flutter
+flutter:
+  fonts: []
+''';
+    final BufferLogger logger = BufferLogger.test();
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, isNotNull);
+    expect(flutterManifest!.fonts.length, 0);
+  });
+
+  testWithoutContext('FlutterManifest ignores empty list of assets', () {
+    const String manifest = '''
+name: test
+dependencies:
+  flutter:
+    sdk: flutter
+flutter:
+  assets: []
+''';
+    final BufferLogger logger = BufferLogger.test();
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, isNotNull);
+    expect(flutterManifest!.assets.length, 0);
+  });
+
   testWithoutContext('FlutterManifest returns proper error when font detail is '
     'not a list of maps', () {
     const String manifest = '''
@@ -1086,6 +1124,25 @@ flutter:
       contains('Cannot find the `flutter.plugin.platforms` key in the `pubspec.yaml` file. '));
   });
 
+  testWithoutContext('FlutterManifest handles empty licenses list', () async {
+    const String manifest = '''
+name: test
+dependencies:
+  flutter:
+    sdk: flutter
+flutter:
+  licenses: []
+''';
+    final BufferLogger logger = BufferLogger.test();
+    final FlutterManifest? flutterManifest = FlutterManifest.createFromString(
+      manifest,
+      logger: logger,
+    );
+
+    expect(flutterManifest, isNotNull);
+    expect(flutterManifest!.additionalLicenses.length, 0);
+  });
+
   testWithoutContext('FlutterManifest can specify additional LICENSE files', () async {
     const String manifest = '''
 name: test