[FIX] web: DebouncedField: notify changes only when changed

Before this rev., if you opened a form view with, e.g. a float
field, that is unset, and then click on edit, then save, it
actually saved the new value for the float field (being 0),
whereas it shouldn't do any RPC as nothing changed.

This is because the former code couldn't detect if the value "0"
in the input is there because the field is still unset or if the
user simply set it to 0.