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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class C extends core::Object {
   field dynamic x = null;
   constructor •() → void
     : super core::Object::•()
     ;
   method m() → dynamic {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, this);
     return new self::Closure#C#m#function::•(#context);
   }
   method f() → dynamic {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, this);
     return new self::Closure#C#f#function::•(#context);
   }
 }
 class Closure#C#m#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#C#m#function::context = context
     ;
@@ skipping 30 matching lines @@
 static method main() → dynamic {
   self::C c = new self::C::•();
   c.x = 41;
   c.m().call(42);
   if(!42.==(c.x))
     throw "Unexpected value in c.x: ${c.x}";
   dynamic result = c.f().call().call();
   if(!42.==(result))
     throw "Unexpected value from c.f()()(): ${result}";
 }