Account for channel pins update `last_pin_timestamp` can be null

Created by: uwx

Summary

Fixes a rare ArgumentNullException on channel pins update.

Details

The last_pin_timestamp event field for the Channel Pins Update event can actually be null, but this is not accounted for in the library.

Changes proposed

• Don't invoke DateTimeOffset.Parse if the last_pin_timestamp property is null.

Notes

I would like to thank the Academy, and Kef#8576 on Discord for finding this one.