Fix NPE when json file reload

[Linary]

Fix #111

Change-Id: I4f12bcd3ca19464823bc52e5d6b408e4592e1cfc

[codecov]

Codecov Report

Merging #112 into master will increase coverage by 7.38%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master    #112      +/-   ##
===========================================
+ Coverage     68.42%   75.8%   +7.38%     
- Complexity      563     617      +54     
===========================================
  Files            63      63              
  Lines          2337    2339       +2     
  Branches        245     245              
===========================================
+ Hits           1599    1773     +174     
+ Misses          605     416     -189     
- Partials        133     150      +17

Impacted Files	Coverage Δ	Complexity Δ
.../baidu/hugegraph/loader/source/AbstractSource.java	88.88% <100%> (+1.38%)	9 <2> (+1)	⬆️
.../com/baidu/hugegraph/loader/util/DataTypeUtil.java	67.46% <0%> (+0.79%)	32% <0%> (ø)	⬇️
...java/com/baidu/hugegraph/loader/util/LoadUtil.java	73.33% <0%> (+2.22%)	12% <0%> (+1%)	⬆️
.../baidu/hugegraph/loader/progress/LoadProgress.java	100% <0%> (+3.22%)	14% <0%> (+1%)	⬆️
...com/baidu/hugegraph/loader/struct/GraphStruct.java	89.58% <0%> (+4.16%)	11% <0%> (+1%)	⬆️
...m/baidu/hugegraph/loader/reader/jdbc/JDBCUtil.java	9.83% <0%> (+9.83%)	3% <0%> (+3%)	⬆️
...du/hugegraph/loader/reader/InputReaderFactory.java	42.85% <0%> (+14.28%)	2% <0%> (ø)	⬇️
.../baidu/hugegraph/loader/task/SingleInsertTask.java	68.57% <0%> (+14.28%)	7% <0%> (+2%)	⬆️
.../hugegraph/loader/serializer/InputSourceDeser.java	71.42% <0%> (+19.04%)	4% <0%> (ø)	⬇️
...baidu/hugegraph/loader/source/jdbc/JDBCVendor.java	32.87% <0%> (+32.87%)	9% <0%> (+9%)	⬆️
... and 3 more

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