nil method Null safety
- dynamic s
In dart2js (in flutter v1.17) a var by default is undefined.
Use this only if you are in version <- 1.17.
So we assure the null type in json convertions to avoid the
"value":value==null?null:value; someVar.nil will force the null type
if the var is null or undefined.
nil taken from ObjC just to have a shorter sintax.
static dynamic nil(dynamic s) => s == null ? null : s;