ReqInt.pos constructor
Constrains data to positive integer numbers (zero included).
blank the error message in case of a null or empty input value.
mal "malformed", the error message for non-integer input values.
neg the error message if an input is negative.
Implementation
ReqInt.pos({String? blank, String? mal, String? neg})
: this._int(Int.pos(mal: mal, neg: neg), blank);