Design display of fixes in check when there is no source

[chriskrycho]

Spun off from #12595: in some cases, there may be a possible fix but no source, e.g. when passing no input at all (see here from an earlier version of the PR than the one that landed). Most likely, those fixes should not be emitted at all. However, this needs a bit of design: maybe it is legitimate in some cases; and/or perhaps the decision should happen upstream to avoid creating the fix at all?