Simplify conversion of empty lists to null.

pkg/analysis_server/lib/src/edit/edit_domain.dart

Index: pkg/analysis_server/lib/src/edit/edit_domain.dart
diff --git a/pkg/analysis_server/lib/src/edit/edit_domain.dart b/pkg/analysis_server/lib/src/edit/edit_domain.dart
index 1b98c3e08ea340e3a96dadd88583039db31f8ecb..c41c65dd014e0380442f91e8ebcf21e7c32a4acf 100644
--- a/pkg/analysis_server/lib/src/edit/edit_domain.dart
+++ b/pkg/analysis_server/lib/src/edit/edit_domain.dart
@@ -6,6 +6,7 @@ library edit.domain;
 
 import 'dart:async';
 
+import 'package:analysis_server/src/collections.dart';
 import 'package:analysis_server/src/analysis_server.dart';
 import 'package:analysis_server/src/constants.dart';
 import 'package:analysis_server/src/protocol.dart' hide Element;
@@ -280,9 +281,7 @@ class _RefactoringManager {
         // create change
         return refactoring.createChange().then((change) {
           result.change = change;
-          if (!refactoring.potentialEditIds.isEmpty) {
-            result.potentialEdits = refactoring.potentialEditIds;
-          }
+          result.potentialEdits = nullIfEmpty(refactoring.potentialEditIds);
           return _sendResultResponse();
         });
       });