can't overstrike \0

[Doug McIlroy]

The construct
\o'\0.'
draws the diagnostic
<standard input>:1: normal or special character expected (got a node)
in groff 1.20.1.  Apparently \0 is not treated as a special character,
as it was in original troff.

Experiment shows that \o' .' fails the same way, which is defensible
because space doesn't have a definite width in all contexts.  Not so
for \0, which has the width of a digit.

(I encountered this in resurrecting a document from 1986; everything
else worked.)