add missing ObjectId validity check

mongo/collection.go

@@ -347,10 +347,12 @@ func getId(item interface{}) bson.ObjectId {
 		}
 		if fieldName != "" {
 			if bsonId, ok := v.FieldByName(fieldName).Interface().(bson.ObjectId); ok {
+				if bsonId.Valid() {
 					return bsonId
 				}
 			}
 		}
+	}
 
 	return bson.NewObjectId()
 }