Bug(java client): When using batch, a null pointer exception may occur

[lengyuexuexuan]

Bug Report

1. What did you do?
   
   in function "commitWaitAllComplete", a null pointer execption occur.

2. What did you see instead?
   

   incubator-pegasus/java-client/src/main/java/org/apache/pegasus/client/FutureGroup.java

   Lines 115 to 118 in f8de6da

   	results.add(
   	Pair.of(
   	new PException("async task #[" + i + "] await failed: " + fu.cause().getMessage()),
   	null));

   
   Add a judge which judge the fu.cause() is null before using it.