In ReferringPagesPlugin, when there's no referrers, the plugin writes ...nobody
To work, this plugin :

1. gets all referers (...refmgr.findReferrers...)
2. then :
   1. filters them if there're any
   2. else print ...nobody

But if you have referrers before filtering but none after filtering, it will write...nothing. Is it really a bug?

To have ...nobody after filtering, instead of else (at line 90), I put if( links == null || links.size() == 0 )

Here's the code :

            ...
            Collection   links  = refmgr.findReferrers( page.getName() );
            ...

            ...
            if( links != null && links.size() > 0 )
            {
                ...
                filtering and more
                ...
            }
            else    <--------------------------Here            
            {
                wikitext = "...nobody";
            }

Fixed in 2.4.31