Change arg of `_.json.Pointer()` to be optional

This method was creating an empty json pointer when user passed a `none`
value, but having to pass `none` made for a bad interface. This patch
allows to simply omit the argument.

sources/Text/JSON/JSONAPI.uc

@@ -79,7 +79,7 @@ private final function InitFormatting()
  *      an incorrect JSON pointer or `none`, - empty `JSONPointer` will be
  *      returned.
  */
-public final function JSONPointer Pointer(Text pointerAsText)
+public final function JSONPointer Pointer(optional Text pointerAsText)
 {
     return JSONPointer(_.memory.Allocate(class'JSONPointer'))
         .Set(pointerAsText);