patch 8.2.5094: MS-Windows GUI: empty command may cause a dialog

Problem:    MS-Windows GUI: empty command may cause a dialog.
Solution:   Delete the dialog file.  Improve the message.
diff --git a/src/testdir/test_ex_mode.vim b/src/testdir/test_ex_mode.vim
index d981ced..fafdda8 100644
--- a/src/testdir/test_ex_mode.vim
+++ b/src/testdir/test_ex_mode.vim
@@ -261,6 +261,9 @@
   call assert_equal(1, RunVim([], [], '-u NONE -e -s -S Xexmodescript'))
 
   call delete('Xexmodescript')
+
+  " This may cause a dialog to be displayed for an empty command, ignore it.
+  call delete('guidialogfile')
 endfunc