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

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

 library;
 import self as self;
 import "dart:core" as core;
 import "dart:mock" as mock;
 
 class C1 extends core::Object {
   field dynamic x;
   constructor •(dynamic y) → void
     : self::C1::x = let final mock::Context #context = new mock::Context::•(1) in let dynamic #t1 = #context.[]=(0, y) in new self::Closure#C1##function::•(#context), super core::Object::•()
     ;
 }
 class C2 extends core::Object {
   field dynamic x = null;
   constructor •(dynamic y) → void
     : super core::Object::•() {
     final mock::Context #context = new mock::Context::•(1);
-    #context.parent = null;
     #context.[]=(0, y);
     this.x = new self::Closure#C2#function#function::•(#context);
   }
 }
 class Closure#C1##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#C1##function::context = context
     ;
@@ skipping 12 matching lines @@
   method call() → dynamic {
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#C2#function#function::context};
     return core::print("Hello ${#contextParameter.[](0)}");
   }
 }
 static method main() → dynamic {
   new self::C1::•("hest").x();
   new self::C2::•("naebdyr").x();
 }