Possible divide-by-zero exception when closing BatchWriter

TabletServerBatchWriter.logStats will throw a divide-by-zero exception if numBatches is 0 (i.e., no batches have been written).

There are actually no checks for 0 divisors anywhere in the method, so other such exceptions are possible, as are lesser issues ("Average send rate" and some other stats come out as NaN when no batches have been written, for example).