HFileReader get split point handle empty HFile better

currenttly if we call org.apache.hadoop.hbase.io.hfile.HFileReaderImpl#midKey on an empty HFile, we got an exception like

 java.io.IOException: HFile empty

since the function return an Optional<Cell> , I think it's better return Optional.empty() instead of throw An Exception

when a region with muiltiple column family grow big enough to be splited, if got some empty column family, will got some warn log like this

java.io.IOException: HFile empty at org.apache.hadoop.hbase.io.hfile.HFileBlockIndex$BlockIndexReader.midkey(HFileBlockIndex.java:334)

sinece exception is catched,  split logic will go on and got right result