fix: guard self.unit against None in Job.__repr__

[Jo2234]

Summary

Guard self.unit against None in Job.__repr__() to prevent a TypeError crash.

Problem

When self.unit is None (its initial value at line 233) and self.interval == 1, calling repr() or str() on the Job crashes:

>>> import schedule
>>> job = schedule.every(1)
>>> repr(job)
TypeError: 'NoneType' object is not subscriptable

This happens because self.unit[:-1] is called at lines 305 and 320 without first checking if self.unit is None.

Fix

-  self.unit[:-1] if self.interval == 1 else self.unit,
+  self.unit[:-1] if self.unit and self.interval == 1 else self.unit,

Applied to both occurrences (lines 305 and 320).

Fixes #646