fix for merging newly created empty TDigest

fix for the following exception:
java.util.NoSuchElementException: Can't iterate past end of data
	at com.tdunning.math.stats.GroupTree$1.next(GroupTree.java:238)
	at com.tdunning.math.stats.GroupTree$1.next(GroupTree.java:212)
	at com.tdunning.math.stats.AbstractTDigest.merge(AbstractTDigest.java:69)