format_entry appended "* N/A = Too few employees to publish (privacy)"
under every category table, including one where every row has an index,
so the output asserted a suppression that never happened - the residual
noted on #470. Set a flag in the N/A* branch and print the footnote only
when it fired; a table with a suppressed row renders exactly as before.
Two FormatEntryTests cases pin both directions; the "omitted" case fails
on master.