Missing docs and tests for _cat/segments

[javanna]

We added _cat/segments api in #4711, but its docs are missing and we need to add some REST tests for it.

[cbuescher]

Hi, writing the Rest-Tests, I realized the _cat/segments command raises an "IndexMissingException" on an empty cluster where other commands like 'shards', 'aliases' etc... return silently. Want me to change that as well or test for the 404 error?

[javanna]

Hey @cbuescher good point! I'd say keep it consistent with other cat apis, it makes sense not to return an exception if the cluster has no indices. Maybe send a separate PR for this change, so we can isolate it since we might need to mark it as breaking.

[cbuescher]

Hi @javanna, do you think I should open a new issue for the possible change in behaviour when there are no indices or just open another pull request for that. If so, use which issue number for that? Just asking to get to know your process better.

[javanna]

Hi @cbuescher I would just open a new PR for that specific change, we don't necessarily need an issue for it (PR is the issue in that case).