JBTM-1790. Check if xaNodeName received from TXControl is not null.

I've added simple checking or exceptions in the places where null pointer exception can be thrown by using TxControl.getXANodeName(). It works for all JTS tests which throw NullPointerException after setting node name to null.

However, there are other places which utilise node names as null, but the do the checking for that, so I didn't touch that.