Null mergeCombiners will cause NPE in ExternalAppendOnlyMap

in most time , null mergeCombiners works fine if each key is different
from others, but if some key are duplicate , it will cause NPE then the
mergeIfKeyExists is called in ExternalAppendOnlyMap.scala
repair the type mistake from V to Any

repair the type mistake from V to Any
Repair another null mergeCombiner risk

Repair another null mergeCombiner risk
Merge pull request #297 from tianyi/master

Null mergeCombiners will cause NPE in ExternalAppendOnlyMap