model: register the implicitly injected Bool class in its hierarchy.

Signed-off-by: Jean Privat <jean@pryen.org>
niti: do not create `true` and `false` if Bool does not exists

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_empty_module2.nit

Signed-off-by: Jean Privat <jean@pryen.org>