Nil-UUID safe-guards

[bounoable]

goes currently doesn't distinguish between Nil-UUIDs and valid UUIDs. This might lead to problems, notably within aggregates and projections, where Nil-UUIDs are treated as legitimate.

To tackle this, we could introduce protective measures against Nil-UUIDs in both the aggregate and event packages. This change could lead to backwards incompatibility, but the potential for reducing errors and improving overall system robustness makes it seem worthwhile.