Event color cannot be cleared because colorId = null is omitted during serialization #324

Describe the bug
Commit d79b0e2 in #322 introduced a regression when updating calendar events. The Nylas documentation states that setting colorId to null should clear the event color. However, the Java SDK omits null fields during Jackson serialization, so colorId is not included in the request payload and the existing color remains unchanged.

As a result, there is currently no documented way to clear an event color using the Java SDK.

To Reproduce

1. Create or update an event with a non-null colorId.
2. Update the same event with colorId = null.
3. Observe that the event color is not cleared because the colorId field is omitted from the serialized JSON payload.

Expected behavior
When colorId is set to null, the SDK should send "color_id": null so that the Nylas API can clear the event color as documented.

SDK Version:
#322

Additional context
There is undocumented Google-specific behavior that will clear the color when setting color_id equal to "0".

There are two possible approaches to resolve this issue:

1. Update the SDK documentation to note that setting colorId = "0" will clear colors for Google calendars - although not officially supported by Google documentation.
2. Modify the SDK serialization logic to allow explicit null values to be sent for fields where null has semantic meaning.

The second approach seems like the better long-term solution, since other fields may also require explicit null values in future API updates. The first approach is obviously a trivial update

I am curious which path the maintainers would like to pursue.