fix(ui): don't send empty grid_line with redrawdebug=compositor (#23899)

test/functional/ui/screen_basic_spec.lua

@@ -1096,3 +1096,18 @@ it('CTRL-F or CTRL-B scrolls a page after UI attach/resize #20605', function()
   feed('<C-F>')
   eq(953, funcs.line('w0'))
 end)
+
+it("showcmd doesn't cause empty grid_line with redrawdebug=compositor #22593", function()
+  clear()
+  local screen = Screen.new(30, 2)
+  screen:set_default_attr_ids({
+    [0] = {bold = true, foreground = Screen.colors.Blue},
+  })
+  screen:attach()
+  command('set showcmd redrawdebug=compositor')
+  feed('d')
+  screen:expect{grid=[[
+    ^                              |
+                       d          |
+  ]]}
+end)