vim-patch:9.0.1919: Wrong curswant when clicking on empty line or with vsplits

Problem:  Wrong curswant when clicking on empty line or with vsplits.
Solution: Don't check for ScreenCols[] before the start of the window
          and handle empty line properly.

closes: vim/vim#13132

03cd697d63

test/functional/ui/multigrid_spec.lua

@@ -2307,6 +2307,7 @@ describe('ext_multigrid', function()
       {1:~                                                                               }|
     ]]}
 
+    -- XXX: mouse_check_grid() doesn't work properly when clicking on grid 1
     meths.input_mouse('left', 'press', '', 1, 6, 20)
     -- TODO(bfredl): "batching" input_mouse is formally not supported yet.
     -- Normally it should work fine in async context when nvim is not blocked,