fix BatchMapperIterator stopped when fetched none in the middle batch

[javeme]

Change-Id: I275cf41016829d2594f728c89d66526a7f9d4374

[codecov]

Codecov Report

Merging #64 (46c9e3a) into master (c80f959) will increase coverage by 0.09%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master      #64      +/-   ##
============================================
+ Coverage     84.73%   84.83%   +0.09%     
- Complexity      756      759       +3     
============================================
  Files            59       59              
  Lines          2182     2183       +1     
  Branches        321      321              
============================================
+ Hits           1849     1852       +3     
- Misses          189      190       +1     
+ Partials        144      141       -3     

Impacted Files	Coverage Δ	Complexity Δ
.../baidu/hugegraph/iterator/BatchMapperIterator.java	82.92% <80.00%> (+2.92%)	17.00 <0.00> (+1.00)
.../com/baidu/hugegraph/iterator/WrappedIterator.java	79.41% <100.00%> (ø)	13.00 <0.00> (ø)
...c/main/java/com/baidu/hugegraph/util/DateUtil.java	76.00% <0.00%> (-8.00%)	9.00% <0.00%> (-1.00%)
.../main/java/com/baidu/hugegraph/event/EventHub.java	81.94% <0.00%> (+1.38%)	27.00% <0.00%> (+1.00%)
...n/java/com/baidu/hugegraph/concurrent/KeyLock.java	80.39% <0.00%> (+1.96%)	26.00% <0.00%> (+1.00%)
.../main/java/com/baidu/hugegraph/perf/Stopwatch.java	87.75% <0.00%> (+2.04%)	21.00% <0.00%> (+1.00%)

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[houzhizhen]

remove if, remains while only