Investigate XTS outbound bridge potential nullpointer issue

During working on JBTM-2853 it was found `NullPointer` could be potentially thrown from `OutboundBridgeManager` when `getTransaction()` returns `null`. It's at time when thread is not associated with any txn.

Discussion about this at PR: https://github.com/jbosstm/narayana/pull/1133

```
Transaction transaction = (Transaction)TransactionManager.transactionManager().getTransaction();
```

https://github.com/jbosstm/narayana/blob/master/txbridge/src/main/java/org/jboss/jbossts/txbridge/outbound/OutboundBridgeManager.java#L69

Goal is to investigate this potential exception being trouble or not and if such thing could happen or not.