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

pkg/kernel/testcases/closures/non_void_context.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
@@ skipping 14 matching lines @@
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#main#function#1::context};
     for (; let final dynamic #t5 = #contextParameter in let final dynamic #t6 = 0 in let final dynamic #t7 = self::v in let final dynamic #t8 = #t5.[]=(#t6, #t7) in #t7; ) {
       self::v = false;
     }
   }
 }
 static field dynamic v = null;
 static method main(dynamic arguments) → dynamic {
   final mock::Context #context = new mock::Context::•(1);
-  #context.parent = null;
   #context.[]=(0, null);
   new self::Closure#main#function::•(#context).call(87);
   if(!self::v.==(87)) {
     throw "Unexpected value in v: ${self::v}";
   }
   if(!#context.[](0).==(87)) {
     throw "Unexpected value in w: ${#context.[](0)}";
   }
   self::v = true;
   new self::Closure#main#function#1::•(#context).call();
   if(!self::v.==(false)) {
     throw "Unexpected value in v: ${self::v}";
   }
   if(!#context.[](0).==(false)) {
     throw "Unexpected value in w: ${#context.[](0)}";
   }
 }