Before this commit, when a date field with the remaining_days widget did not have a value
typically on a new record form view, the props validation crashed because the datePicker widget
received the wrong type for its value.
After this commit, this is corrected and there is no crash.
closesodoo/odoo#107233
Related: odoo/enterprise#34668
Signed-off-by: Lucas Perais (lpe) <lpe@odoo.com>