Fix problem with uninitialized unions in _from_dict

test/addressbook.capnp

@@ -21,13 +21,18 @@ struct Person {
 
   employment :union {
     unemployed @4 :Void;
-    employer @5 :Text;
+    employer @5 :Employer;
     school @6 :Text;
     selfEmployed @7 :Void;
     # We assume that a person is only one of these.
   }
 }
 
+struct Employer {
+  name @0 :Text;
+  boss @1 :Person;
+}
+
 struct AddressBook {
   people @0 :List(Person);
 }