fix: issues with command line if ui elements are externalized

Resolve https://github.com/neovim/neovim/issues/20888 and handle side
effects for setting 'cmdheight' to zero.

src/nvim/ex_getln.c

@@ -3841,7 +3841,7 @@ void compute_cmdrow(void)
     cmdline_row = wp->w_winrow + wp->w_height
                   + wp->w_hsep_height + wp->w_status_height + global_stl_height();
   }
-  if (cmdline_row == Rows) {
+  if (cmdline_row == Rows && p_ch > 0) {
     cmdline_row--;
   }
   lines_left = cmdline_row;

src/nvim/ui.c

@@ -235,7 +235,7 @@ void ui_refresh(void)
   }
 
   if (ext_widgets[kUIMessages]) {
-    p_ch = 0;
+    set_option_value("cmdheight", 0L, NULL, 0);
     command_height();
   }
   ui_mode_info_set();