Add `null` named capture coercion test case

test/built-ins/RegExp/prototype/Symbol.replace/result-coerce-groups-prop.js

@@ -58,6 +58,7 @@ var coercibleValue = {
         throw new Test262Error('This method should not be invoked.');
       },
     },
+    bar: null,
   },
 };
 
@@ -66,3 +67,4 @@ r.exec = function() {
 };
 
 assert.sameValue(r[Symbol.replace]('ab', '[$<foo>]'), '[toString value]b');
+assert.sameValue(r[Symbol.replace]('ab', '[$<bar>]'), '[null]b');