Fix entitlement starts_at and ends_at erroring when null

[julien777z]

Summary

Entitlement starts_at and ends_at are nullable in the Discord API, and if it's null, results in this error:

ends_at=time.iso8601_datetime_string_to_datetime(payload["ends_at"]) if "ends_at" in payload else None,
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/python3.12/site-packages/hikari/internal/time.py", line 81, in slow_iso8601_datetime_string_to_datetime
    if datetime_str.endswith(("z", "Z")):
        ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'endswith'

The error for starts_at is similar.

Checklist

• I have run nox and all the pipelines have passed.
• I have made unittests according to the code I have added/modified/deleted.

Related issues

[davfsa]

Thanks!

Sorry that it took me so long to get to this