Provide default message when no issues are found

Currently (v. 9.0) if the query finds no issues, nothing is displayed in page, like no component is even present.

It could be useful to be able to provide a default message (wiki syntax?) as a macro parameter, to be shown in that case.

This suggestion came from the fact that XWiki Release Notes pages have a Regressions block generally commented out, and it could seldom happen that is stays that way, letting regressions live unnoticed (e.g.: https://forum.xwiki.org/t/regressions-for-16-10-2-do-not-show-up-in-release-notes-page/16441).