public form setup does not work when "not synced" properties are created as required.

[daneryl]

Steps to reproduce.

• Create template on private instance
• Configure sync of this template to a public instance
• Config the template as a public form on the public instance
• Create a new required property for the template on the private instance
• Try to send the public form and you get a generic error (a validation error shows on the request tab)

related #4286