fix: missing events in the event store (#4335)

## About the changes
We are losing some events because of not having "created by" which is
required by a constraint in the DB. One scenario where this can happen
is with the default user admin, because it doesn't have a username or an
email (unless configured by the administrator).

Our code makes assumptions on the existence of one of these 2 attributes
(e.g.
248118af7c/src/lib/services/user-service.ts (L220-L222)).

Event lost metrics:
![Screenshot from 2023-07-24
14-17-20](https://github.com/Unleash/unleash/assets/455064/9b406ad0-bbcb-4263-98dc-74ddd307a5a2)


## Discussion points
The solution proposed here is falling back to a default value. I've
chosen `"admin"` because it covers one of the use cases, but it can also
be `"system"` mimicking
248118af7c/src/lib/services/user-service.ts (L32)
which is used as a default, or `"unknown"` which is sometimes used as a
default:
248118af7c/src/lib/services/project-service.ts (L57)

Anyway, I believe it's better not to lose the event rather than be
accurate with the "created by" that can be fixed later

src/lib/db/event-store.ts

@@ -371,7 +371,7 @@ class EventStore implements IEventStore {
     eventToDbRow(e: IBaseEvent): Omit<IEventTable, 'id' | 'created_at'> {
         return {
             type: e.type,
-            created_by: e.createdBy,
+            created_by: e.createdBy ?? 'admin',
             data: Array.isArray(e.data) ? JSON.stringify(e.data) : e.data,
             pre_data: Array.isArray(e.preData)
                 ? JSON.stringify(e.preData)