[result4k] Introduce a way to early return on successful result with a null value

[th3n3rd]

In cases were is preferred to return a nullable type over using a failure to model the absence of a value, would be useful to have a more cohesive way to have an early return that abort the current function.

e.g.

val productDetails = productCatalogue.findById(123)
    .onFailure { return it }

if (productDetails == null) {
    return Success(Unit) // i.e. no-op
}

// ...

can now be written as

val productDetails = productCatalogue.findById(123)
    .onNull { return Success(Unit) } // i.e. no-op
    .onFailure { return it }

// ...

[daviddenton]

It's a good change. Thanks @th3n3rd 🙃