Function isNull

isNull<T>(defaultValue: T): Decoder<T>
Decoder that accepts null values and returns a default value.
Type Parameters
- T
Parameters
- defaultValue: T
  The value to return when null is encountered
Returns Decoder<T>
A decoder that accepts null and returns the default value
Example
```
const numberOrZero = JsonDecoder.isNull(0);

numberOrZero.decode(null); // Ok<number>({value: 0})
numberOrZero.decode(42); // Err({error: '42 is not null'})
```