fix: make avoid-redundant-async correctly handle nullable return values (#1009)

Author: incendial

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## 4.19.1
 
+* fix: make [`avoid-redundant-async`](https://dartcodemetrics.dev/docs/rules/common/avoid-redundant-async) correctly handle nullable return values.
 * fix: make [`avoid-wrapping-in-padding`](https://dartcodemetrics.dev/docs/rules/flutter/avoid-wrapping-in-padding) trigger only on Container widget.
 
 ## 4.19.0

lib/src/analyzers/lint_analyzer/rules/rules_list/avoid_redundant_async/avoid_redundant_async_rule.dart

@@ -3,6 +3,7 @@
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:analyzer/dart/element/nullability_suffix.dart';
 
 import '../../../../../utils/node_utils.dart';
 import '../../../lint_utils.dart';

lib/src/analyzers/lint_analyzer/rules/rules_list/avoid_redundant_async/visitor.dart

@@ -53,7 +53,9 @@ class _AsyncVisitor extends RecursiveAstVisitor<void> {
 
     final type = node.expression?.staticType;
 
-    if (type == null || !type.isDartAsyncFuture) {
+    if (type == null ||
+        !type.isDartAsyncFuture ||
+        type.nullabilitySuffix == NullabilitySuffix.question) {
       hasValidAsync = true;
     }
   }

test/src/analyzers/lint_analyzer/rules/rules_list/avoid_redundant_async/examples/example.dart

@@ -30,4 +30,8 @@ class SomeClass {
 
     print(records);
   }
+
+  Future<void> returnNullable(SomeClass? instance) async {
+    return instance?.report([]);
+  }
 }