For the following 3 functions, previously the code relied on whether the
arg was passed at all, to make it optional. Now it allows them to be
explicitly `None` to indicate they are not used:
- gatts_notify(..., [data])
- gattc_discover_services(..., [uuid])
- gattc_discover_characteristics(..., [uuid])
Also ensure that the uuid arguments are actually instances of the uuid
type, and fix handling of the 5th arg in gattc_discover_characteristics().
