XATerminatorImple#recover could return an empty array in preference to null

The javadoc (http://docs.oracle.com/javaee/7/api/javax/resource/spi/XATerminator.html#recover-int-) for the XATerminator recover
method says the resource manager should return zero or more XIDs or throw an exception but our implementation of it returns null if there are no in doubt Xids.