UI handler is not defined for the null toolkit

The null toolkit is very useful for testing:
Canned/fake objects can be created using the null toolkit, It helps writing toolkit agnostic code and toolkit agnostic tests.

One barrier to this plan is that the `ui_handler` is not set for the null toolkit, which means any `on_trait_change(..., dispatch="ui")` or `observe(..., dispatch="ui")` will fail at the time of invocation when the null toolkit is used.