Returns a validator function to validate that a radio option has been selected if required.
Note: empty strings and null count as missing a value (invalid if required), all other values are valid.
null
Optional
Indicates if the value is required.
A custom error message to display if the value is invalid.
Returns a validator function to validate that a radio option has been selected if required.
Note: empty strings and
nullcount as missing a value (invalid if required), all other values are valid.