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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class C extends core::Object {
   field dynamic t;
   constructor foo(dynamic f) → void
     : self::C::t = let final mock::Context #context = new mock::Context::•(1) in let dynamic #t1 = #context.[]=(0, f) in new self::Closure#C#foo#function::•(#context), super core::Object::•() {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, f);
     core::print(1);
   }
 }
 class Closure#C#foo#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#foo#function::context = context
     ;
@@ skipping 15 matching lines @@
     return core::print("hest");
   }
 }
 static method main() → dynamic {
   core::print(0);
   dynamic c = new self::C::foo(new self::Closure#main#function::•(null));
   core::print(2);
   c.t();
   core::print(3);
 }