fix(client): Return nil and error if endpoint is empty string

[akosiaris]

Why: Passing an empty string as an endpoint to Client when instantiating a new client might seem like something that should never happen but I managed to trigger it while parsing some input files to register feeds in bulk.

What: In the execute() function, check early if the endpoint is "" and then return immediately nil and a new error, named ErrWrongEndpoint with a descriptive string

Do you follow the guidelines?

• [x] I have tested my changes
• [x] There is no breaking changes
• [x] I really tested my changes and there is no regression
• [x] Ideally, my commit messages follow the Conventional Commits specification
• [x] I read this document: https://miniflux.app/faq.html#pull-request

[akosiaris]

Taking a quick look, I think your solution is better than mine. As long as no Client's endpoint field is used outside of a request context, it shoud work fine.

I 'll try and code it, run it for a few days and then get back to you.

Thanks for the suggestion!

[akosiaris]

Updated per your suggestion. I came up with the error name and string but happy to alter name and wording.