Eskip check fails on empty URL on network backend.

[xavivars]

This change to eskip check validates no routes have empty backend.

"empty url" error falls between the "pure syntactical" checks that eskip check runs at validation time and the "route-specific setup-dependant" checks that skipper does at runtime (making sure, for example, that all predicates / filters exist, etc).

While this is not a fatal error, and loading the skip files in skipper works (only the specific route gets discarded), we think having this validation in eskip check is a better approach, because this is an error that is known in advance (no URL can be empty) and we could not find a reason we'd ever want to, on purpose, push a non-working route to production.

Fixes #2523

[AlexanderYastrebov]

Thank you.
I think we can have it like you propose (after comments are addressed).

In general I think we should make backend validation (both network and loadbalanced) more strict during eskip parsing (reject empty values, non-parseable urls, urls with non-empty path, etc) - then k8s dataclient and webhook would benefit from it as well. @MustafaSaber is working on validation topic, see related #1618

[xavivars]

Thanks for the quick review!

I think I've addressed all your comments now.

And I agree: stricter backend validation would be great.

[AlexanderYastrebov]

Thank you.

[AlexanderYastrebov]

👍

[szuecs]

👍