fix(events): handle nil auth when building event doers

ProjectUser.Create and friends are called with a nil auth in tests;
the old interface-typed Doer just serialized as null, so a nil doer
keeps that behavior (and maps to the system actor in the audit entry).

pkg/models/users.go

@@ -27,6 +27,9 @@ import (
 // flows acting on behalf of disabled accounts (e.g. user deletion), and the
 // event only needs the principal as it authenticated.
 func doerFromAuth(a web.Auth) *user.User {
+	if a == nil {
+		return nil
+	}
 	if u, is := a.(*user.User); is {
 		return u
 	}