closure conversion: Do not set Context.parent if the value is `null`

pkg/kernel/testcases/closures/type_variables.dart.expect

Index: pkg/kernel/testcases/closures/type_variables.dart.expect
diff --git a/pkg/kernel/testcases/closures/type_variables.dart.expect b/pkg/kernel/testcases/closures/type_variables.dart.expect
index b58d4db252fe95f979b21becd0efd3a93e5de9b8..45d06e3f4a2dfab6ce713f0c2ed234e15928445a 100644
--- a/pkg/kernel/testcases/closures/type_variables.dart.expect
+++ b/pkg/kernel/testcases/closures/type_variables.dart.expect
@@ -9,7 +9,6 @@ class C<T extends core::Object, S extends core::Object> extends core::Object {
     ;
   method foo(self::C::S s) → dynamic {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, this);
     return new self::Closure#C#foo#function::•<self::C::T, self::C::S>(#context);
   }