Use null aware operators to clean up code

pkg/analysis_server/lib/src/computer/computer_hover.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 library computer.hover;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     show HoverInformation;
 import 'package:analysis_server/src/computer/computer_overrides.dart';
 import 'package:analysis_server/src/utilities/documentation.dart';
@@ skipping 109 matching lines @@
       String rawDoc = superElement.documentationComment;
       if (rawDoc != null) {
         Element interfaceClass = superElement.enclosingElement;
         return removeDartDocDelimiters(rawDoc) +
             '\n\nCopied from `${interfaceClass.displayName}`.';
       }
     }
     return null;
   }
 
-  static _safeToString(obj) => obj != null ? obj.toString() : null;
+  static _safeToString(obj) => obj?.toString();
 }