l10n.yaml's nullable-getter option should default to true (#124353)

Currently, nullable-getter defaults to false when l10n.yaml is not present, which is not the same behavior as when an l10n.yaml file is present and nullable-getter is not set.

Fixes #120457.

packages/flutter_tools/lib/src/commands/generate_localizations.dart

@@ -179,6 +179,7 @@ class GenerateLocalizationsCommand extends FlutterCommand {
     );
     argParser.addFlag(
       'nullable-getter',
+      defaultsTo: true,
       help: 'Whether or not the localizations class getter is nullable.\n'
             '\n'
             'By default, this value is set to true so that '

packages/flutter_tools/test/commands.shard/hermetic/generate_localizations_test.dart

@@ -319,4 +319,36 @@ untranslated-messages-file: lib/l10n/untranslated.json
     FileSystem: () => fileSystem,
     ProcessManager: () => FakeProcessManager.any(),
   });
+
+  testUsingContext('nullable-getter defaults to true', () async {
+    final File arbFile = fileSystem.file(fileSystem.path.join('lib', 'l10n', 'app_en.arb'))
+      ..createSync(recursive: true);
+    arbFile.writeAsStringSync('''
+{
+  "helloWorld": "Hello, World!",
+  "@helloWorld": {
+    "description": "Sample description"
+  }
+}''');
+    final File pubspecFile = fileSystem.file('pubspec.yaml')..createSync();
+    pubspecFile.writeAsStringSync(BasicProjectWithFlutterGen().pubspec);
+    final GenerateLocalizationsCommand command = GenerateLocalizationsCommand(
+      fileSystem: fileSystem,
+      logger: logger,
+      artifacts: artifacts,
+      processManager: processManager,
+    );
+    await createTestCommandRunner(command).run(<String>['gen-l10n']);
+
+    final Directory outputDirectory = fileSystem.directory(fileSystem.path.join('.dart_tool', 'flutter_gen', 'gen_l10n'));
+    expect(outputDirectory.existsSync(), isTrue);
+    expect(outputDirectory.childFile('app_localizations.dart').existsSync(), isTrue);
+    expect(
+      outputDirectory.childFile('app_localizations.dart').readAsStringSync(),
+      contains('static AppLocalizations? of(BuildContext context)'),
+    );
+  }, overrides: <Type, Generator>{
+    FileSystem: () => fileSystem,
+    ProcessManager: () => FakeProcessManager.any(),
+  });
 }