An empty host is not detected by validation

[Hemiun]

Version of KrakenD you are using
2.8.0

Is your feature request related to a problem? Please describe.
There isn't some validation for missed host in backend section of the configuration. We only find out about the problem after the first call. According to the documentation, the service should not be running:

An array with all the available hosts to [load balance](https://www.krakend.io/docs/throttling/load-balancing/#balancing-egress-traffic-to-upstream) requests, including the schema (when possible) schema://host:port. E.g.: https://my.users-ms.com. If you are in a platform where hosts or services are balanced (e.g., a K8S service), write a single entry in the array with the service name/balancer address. Defaults to the host declaration at the configuration’s root level, and the service fails starting when there is none.

In fact, the service starts without any worning or error.
I belive that the host is critical to launching the service and the user user should be aware of the problem as early as possible and not during query execution.

Describe the solution you'd like
I propose

• Consider adding a warning at startup time
• Consider implementing more stringent verification of the configuration (check command)
• Describe actual behavior in documentaion

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
configuration example
Steps:

1. Check config

krakend check -c krakend.json
Syntax OK!

2. Run gateway

krakend run -c krakend.json

3. Send request

GET http://<address>/api-without-backend
[GIN] 2024/12/13 - 16:53:26 | 500 |     264.166µs |   <address> | GET      "/api-without-backend"

[thedae]

Hi @Hemiun thanks for reporting this issue, great catch!

We've logged this internally and will be working on a fix soon. You can expect this to be addressed as a patch release.