don't trip over missing create events when loading power levels

as the test from previous commit fails because of that, and powerlevels
works fine without

src/matrix/room/timeline/Timeline.js

@@ -84,10 +84,14 @@ export class Timeline {
             });
         }
         const createState = await txn.roomState.get(this._roomId, "m.room.create", "");
+        if (createState) {
             return new PowerLevels({
                 createEvent: createState.event,
                 ownUserId: this._ownMember.userId
             });
+        } else {
+            return new PowerLevels({ownUserId: this._ownMember.userId});
+        }
     }
 
     _setupEntries(timelineEntries) {