[FIX] web_editor: uncheck new checklist item on split

On ENTER in a checklist item, a new checklist item is created. The new
one should never be checked. This ensures that ENTER in a checked
checklist item never creates a checked checklist item but instead cleans
the class.

closes odoo/odoo#32684

Signed-off-by: Pierre Paridans <pparidans@users.noreply.github.com>