'main': Fix highlighting of null execs.

Fixes #676.

highlighters/main/main-highlighter.zsh

@@ -824,6 +824,10 @@ _zsh_highlight_main_highlighter_highlight_list()
       next_word=${next_word//:regular:/}
       next_word+=':sudo_opt:'
       next_word+=':start:'
+      if [[ $arg == 'exec' ]]; then
+        # To allow "exec 2>&1;" where there's no command word
+        next_word+=':regular:'
+      fi
      else
       case $res in
         reserved)       # reserved word