Handle empty input

[psfinaki]

Problem

Currently the service will throw because of let firstLetter = artist.[0] in the UrlMaker.

Ideas

Time to introduce some kind of validation to the service. Then, maybe, to the client as well.

[psfinaki]

Now the error is generated at the service in the cases of null, empty or whitespace inputs.

For this, a new ErrorMessage called EmptyInput is introduced in the Result type. Validation itself is done in the new Validator module. It is well-tested in the ValidationTests module.