fix(containment): return nil for actor shapes without an id

Containment.get_actor raised on remote shapes it did not know, such as
an attributedTo object without an id. This crashed object fetches and
poll refetches. Return nil, so containment rejects the object.

changelog.d/get-actor-crash.fix

@@ -0,0 +1 @@
+Reject fetched objects whose author has no id instead of crashing