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

pkg/kernel/testcases/closures/catch.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 {
     "This is a temporary solution. In the VM, this will become an additional parameter.";
     final mock::Context #contextParameter = this.{self::Closure#main#function::context};
     core::print(#contextParameter.[](0));
     if(!#contextParameter.[](1).==(null))
       core::print(#contextParameter.[](1));
   }
 }
 static method main() → dynamic {
   dynamic c;
   try {
     throw "Fisk";
   }
   on core::String catch(dynamic #t1, dynamic #t2) {
     final mock::Context #context = new mock::Context::•(2);
-    #context.parent = null;
     #context.[]=(0, #t1);
     #context.[]=(1, #t2);
     c = new self::Closure#main#function::•(#context);
   }
   c.call();
   core::print("TEST PASSED");
 }