[FIX] website_form: Required hidden field is not saved

Issue

	- Install "Website" app
	- Go to "Contact us" page on website
	- Edit Page, then click on submit button to
	  edit form style
	- Alter 'Recipient Email' then save
	- Edit Page and click on submit button

	'Recipient Email' have default value.

Cause

	If a field is required, the default value is set without
	checking if a value is already set.

Solution

	Set current value if available, else default value.

opw-2361169

closes odoo/odoo#60452

X-original-commit: e29d2818abf0e7902feb26c9e0b4809c9a3351c6
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>
Signed-off-by: bon-odoo <nboulif@users.noreply.github.com>