[luatex] Bug: Null-byte in csname not handled correctly

[Henri Menke]

Dear devs,

A control sequence that only contains a single null byte is not
correctly stringified. Comparison between Knuth TeX and LuaTeX:

   $ tex test.tex
   ! \^^@.
   l.2 \errmessage{\string\^^@}
   
   $ luatex test.tex
   ! \.
   l.2 \errmessage{\string\^^@}

MWE:

   \catcode0=11
   \errmessage{\string\^^@}
   \bye

Cheers,
Henri