fix(cmdline): encode no prompt in cmdline_show.hl_id

Problem:  Unable to distinguish an empty prompt from no prompt in
          cmdline_show event.
Solution: Set cmdline_show.hl_id to -1 when no prompt is active.

test/functional/ui/screen.lua

@@ -1390,7 +1390,7 @@ function Screen:_handle_cmdline_show(content, pos, firstc, prompt, indent, level
   if firstc == '' then
     firstc = nil
   end
-  if prompt == '' then
+  if hl_id == -1 then
     prompt = nil
   end
   if indent == 0 then