Steps to reproduce:
- set wiki to multi-language
- search for "Sandbox"
- open the "Language" facet and select only for pages with "No Language"
This creates three search results: Sandbox.TestPage1,2,3
I guess these pages should have an explicit defaultLanguage=en (unlike most pages in platform, who are indeed not in any Language, but like the most of the page in xwiki-platform-help-ui).
What I am not so sure about is if that change will create lots of conflicts when upgrading the content ... in that case it might not worth the change.