Add default value for swagger 200 response

[johnchildren]

Re: #766

This commit adds a default value to the description field of 200
responses. This is needed to meet the swagger specification requirement
of a short description for all response objects.

[johnchildren]

I couldn't see an obvious place to add this in the tests, but they seemed to pass locally regardless of the change. It may be worth also updating the example swagger files as they currently have empty descriptions.

edit: based on CI I was obviously running the wrong command, will look into updating the generated files.

[johanbrandhorst]

@ivucica would like your opinion on this if possible.

[johanbrandhorst]

You needn't really bother with tests, it should be good enough that the generated files now have a description set somewhere where it wasn't before.

[ivucica]

Regarding justification: I'd argue the spec only requires a string to be set, and not that it's a non-empty string. See relevant section in spec; while it says it's required, it doesn't say what the string should look like.

(For what it's worth, the spec also actually doesn't say what it means by the word "required" in this context; the quoted RFCs use it slightly differently.)

editor.swagger.io does not complain about a description set to an empty string.

But, having said this, I believe the change to be an improvement. LGTM.

On a separate note: line 717 will make the resulting description look ugly, but that's already the case if people specify a documentation comment. I'd remove that case, but that's something to be done in a separate PR.