Fix model syncing in widget editor

- [x] null and undefined should be considered the same and shouldn't be synced (setting undefined in a model or editor might go through some castings and end up as null when asked for again, thus cause infinite loops when syncing the model with the form)
- [x] `getValue` should resort to `@default_value` when the widget is missing (not yet picked up by the widget starter) and not when it returns a _false_ value (see line 94)