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

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

Index: pkg/kernel/testcases/closures/capture_closure.dart.expect
diff --git a/pkg/kernel/testcases/closures/capture_closure.dart.expect b/pkg/kernel/testcases/closures/capture_closure.dart.expect
index 830004b27c931eb013f4758089ab6c37cbb2508e..2aa69a182fd9f5291e3793d854f2b845a1fedd10 100644
--- a/pkg/kernel/testcases/closures/capture_closure.dart.expect
+++ b/pkg/kernel/testcases/closures/capture_closure.dart.expect
@@ -29,7 +29,6 @@ class Closure#main#g extends core::Object implements core::Function {
 }
 static method main(dynamic arguments) → dynamic {
   final mock::Context #context = new mock::Context::•(1);
-  #context.parent = null;
   #context.[]=(0, new self::Closure#main#f::•(#context));
   final () → dynamic g = new self::Closure#main#g::•(#context);
   g.call();