role::icon and role::unicode_emoji should be marked as optional

[Nihlus]

Description
The new icon and unicode_emoji properties introduced in #3847 should be marked as optional, in addition to nullable. The API may omit one or the other, depending on the server conditions.

Steps to Reproduce

1. Call the API
2. Observe that icon and/or unicode_emoji may not be present in certain instances

Expected Behavior
Either icon and unicode_emoji should always be present, or the documentation should be updated to reflect their optionality.

Current Behavior
icon and unicode_emoji may not be present in certain instances, contrary to the documented behaviour.