Add `BLI_uuid_nil()` that returns the nil UUID (used to indicate "not set") and `BLI_uuid_is_nil(uuid)` to do an equality test with the nil value.
Add `BLI_uuid_nil()` that returns the nil UUID (used to indicate "not set") and `BLI_uuid_is_nil(uuid)` to do an equality test with the nil value.