$READNULLCMD / $NULLCMD are red

[danielshahaf]

% tests/generate.zsh '$NULLCMD | $READNULLCMD' main nullcmd 'local NULLCMD=cat READNULLCMD=f; f(){};' 
Set copyright year to 2020? y
local NULLCMD=cat READNULLCMD=f; f(){};

BUFFER=$'$NULLCMD | $READNULLCMD'

expected_region_highlight=(
  '1 8 unknown-token' # $NULLCMD
  '10 10 commandseparator' # |
  '12 23 unknown-token' # $READNULLCMD
)

That's likely because they're special, and as such don't get expanded (which makes this another case of #695):

% p ${(t)READNULLCMD} ${(t)NULLCMD}
scalar-special scalar-special