Ignore empty inputs if field isn't required

[fullonic]

Currently, if a form field is not required and leaves it blank, it can raise an error if an empty value string ("") isn't valid, for example, a UUID field.

This PR fixes this issue by checking if the field is required and if not, checks if its value is empty. If the value is empty, the field is ignored.

We need to check what happens with other fields type, where the empty value can be another type than an empty string