Validate on constraint property changes if the field has a value

[vursen]

As illustrated in #4196, field components don't trigger validation consistently on constraint property change.

Let's align their behavior in the following way:

• Trigger validation on constraint property changes if the field has a value (because the value can possibly be invalidated by the new constraints configuration)
• Trigger validation during initialization if at least one constraint is defined and the field has a value (providing an initial value that doesn't match constraints sounds more like an edge case and rather an app business logic error, but if this has happened by any chance, it is better to provide immediate feedback for the user to be able to correct the value)

A related ticket for Flow components: vaadin/flow-components#2370