Don't report avoid_function_literals_in_foreach_calls with null-aware call (dart-archive/linter#4039)

* Don't report `avoid_function_literals_in_foreach_calls` with null-aware call

The current behavior is fully correct in null-safe mode, but in legacy mode the check for nullable type is inverted.

Not adding a test because there is no legacy-mode test file.

Closes dart-lang/linter#3354

* Add test

* Move test to noSoundNullSafety

Author: oprypin

lib/src/rules/avoid_function_literals_in_foreach_calls.dart

@@ -54,7 +54,7 @@ bool _hasMethodChaining(MethodInvocation node) {
 
 bool _isNonNullableIterable(DartType? type) =>
     type != null &&
-    type.nullabilitySuffix != NullabilitySuffix.question &&
+    type.nullabilitySuffix == NullabilitySuffix.none &&
     type.implementsInterface('Iterable', 'dart.core');
 
 class AvoidFunctionLiteralInForeachMethod extends LintRule {

test/rules/avoid_function_literals_in_foreach_calls_test.dart

@@ -2,6 +2,7 @@
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
+import 'package:analyzer/src/utilities/legacy.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import '../rule_test_support.dart';
@@ -98,3 +99,28 @@ void f(List<String> people) {
 ''');
   }
 }
+
+@reflectiveTest
+class AvoidFunctionLiteralsInForeachCallsPreNNBDTest extends LintRuleTest {
+  @override
+  String get lintRule => 'avoid_function_literals_in_foreach_calls';
+
+  @override
+  setUp() {
+    super.setUp();
+    noSoundNullSafety = false;
+  }
+
+  tearDown() {
+    noSoundNullSafety = true;
+  }
+
+  test_functionExpression_nullableTarget() async {
+    await assertNoDiagnostics(r'''
+// @dart=2.9
+void f(List<String> people) {
+  people?.forEach((person) => print('$person'));
+}
+''');
+  }
+}