[CRASH CONDITION] - Guard self.unit against None

https://github.com/dbader/schedule/blob/82a43db1b938d8fdf60103bd41f329e06c8d3651/schedule/__init__.py#L305

• If self.interval == 1 is True, and, self.unit is None (which is what its being initialized to here) : https://github.com/dbader/schedule/blob/3863effe15ec8239ba88c65a4231a67e43b116df/schedule/__init__.py#L232 It results in crash.

• The code should guard against self.unit being None before attempting self.unit[:-1]