[FIX] web: List editable top select last row

Before this commit, when we have a list (editable top) in a form
with some mandatory fields and some existing rows. If we click on
'add a line', a new empty line appear on the top, then we click on
the last row, the empty row will disappear and we will have a
traceback.

After this commit, if we repeat this scenario, we click on the last
row, we will be able to edit this last row.

closes odoo/odoo#38302

Signed-off-by: VincentSchippefilt <VincentSchippefilt@users.noreply.github.com>