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

[juliosuas]

Summary

Fixes #646.

When Job.interval == 1 and self.unit is None (the initial value before a unit is set), __repr__ attempted self.unit[:-1], which raised:

TypeError: 'NoneType' object is not subscriptable

Root Cause

self.unit is initialized to None in Job.__init__:

self.unit: Optional[str] = None

But both format paths in __repr__ unconditionally subscript self.unit:

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

Fix

Guard both paths with a None check, substituting '[unit not set]' as a safe placeholder:

(self.unit[:-1] if self.interval == 1 else self.unit) if self.unit else '[unit not set]'

Verification

job = schedule.Job(interval=1)
repr(job)  # Before: TypeError crash / After: 'Every 1 [unit not set] do [None] ...'

[juliosuas]

Quick note: the fix also handles the else-branch in __repr__ where the same self.unit[:-1] pattern appeared in the format string dict. Both paths now safely fall back to '[unit not set]' when self.unit is None. Normal jobs that have been assigned a unit are completely unaffected.