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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class Closure#main#f extends core::Object implements core::Function {
   field core::String note = "This is temporary. The VM doesn't need closure classes.";
   field mock::Context context;
   constructor •(final mock::Context context) → dynamic
     : self::Closure#main#f::context = context
@@ skipping 11 matching lines @@
     : self::Closure#main#g::context = context
     ;
   method call() → dynamic {
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#main#g::context};
     return #contextParameter.[](0).call();
   }
 }
 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();
 }