Use nil instead of null in json

lib/pure/json.nim

@@ -628,9 +628,6 @@ proc newJArray*(): JsonNode =
   result.kind = JArray
   result.elems = @[]
 
-const
-  null*: string = nil ## alias for ``nil``
-
 proc getStr*(n: JsonNode, default: string = ""): string =
   ## Retrieves the string value of a `JString JsonNode`.
   ##
@@ -1315,5 +1312,5 @@ when isMainModule:
     ]
   doAssert j3 == %[%{"name": %"John", "age": %30}, %{"name": %"Susan", "age": %31}]
 
-  var j4 = %*{"test": null}
+  var j4 = %*{"test": nil}
   doAssert j4 == %{"test": newJNull()}