only call .replace if object exists

[jlamb1]

I came across this error in my logs using the MBTALive 2.0 HACS integration:

Unexpected error in __is_alert_relevant: 'NoneType' object has no attribute 'replace'
Traceback (most recent call last): File "/usr/local/lib/python3.14/site-packages/mbtaclient/handlers/base_handler.py",
line 411, in __is_alert_relevant active_period_end = mbta_alert.active_period_end.replace(tzinfo=None) 
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ AttributeError: 'NoneType' object has no attribute 'replace'

this adds a simple check to only call .replace() if exists.

This may be similar to issue #49