TYPEMAP
maybe_string   T_MAYBE_STRING

OUTPUT
T_MAYBE_STRING
  $var != NULL ? sv_setpv((SV*)$arg, $var) : sv_setsv((SV*)$arg, &PL_sv_undef);

INPUT
T_MAYBE_STRING
  $var = SvOK($arg) ? ($type)SvPV_nolen($arg) : NULL;
