refactor: Check correct if null

This checks it in a more consistent way
and also makes developing new
databases easier as they
might set the json key but
set it null.

lib/src/event.dart

@@ -211,7 +211,7 @@ class Event extends MatrixEvent {
         type: jsonPayload['type'],
         eventId: jsonPayload['event_id'] ?? '',
         senderId: jsonPayload['sender'],
-        originServerTs: jsonPayload.containsKey('origin_server_ts')
+        originServerTs: jsonPayload['origin_server_ts'] != null
             ? DateTime.fromMillisecondsSinceEpoch(
                 jsonPayload['origin_server_ts'])
             : DateTime.now(),