Reports should be queued for reporting observers even when no endpoint exists

[clelland]

In some cases, it may be desirable to produce reports which can be seen by reporting observers, even when no endpoint has been configured.

Permissions-policy violations, for instance, can be observed and reacted to in the running page, but enabling this requires sending an endpoint value into generate and queue a report.

Callers could use a deliberately invalid name, so that the later processing steps will discard the queued report, but it would be better to make it optional in the first place.