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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class Closure#main#function 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#function::context = context
     ;
   method call(dynamic x) → dynamic {
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#main#function::context};
     return #contextParameter.[](0).[](x);
   }
 }
 static field dynamic f = null;
 static method foo() → dynamic {
   core::print(self::f.call(0));
 }
 static method main(dynamic arguments) → dynamic {
   final mock::Context #context = new mock::Context::•(1);
-  #context.parent = null;
   #context.[]=(0, arguments);
   self::f = new self::Closure#main#function::•(#context);
   self::foo();
 }