fix: guard self.unit against None in __repr__

[thakoreh]

Summary

Fixes #646

Problem

When self.interval == 1 and self.unit is None, the code attempts to slice self.unit[:-1], which causes a crash.

TypeError: 'NoneType' object is not subscriptable

Solution

Added a guard to check that self.unit is not None before attempting to slice it:

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

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

This change is applied to both occurrences in the __repr__ method (lines 305 and 320).