fix task result is null even it is COMPLETED

[javeme]

task result is not set when set status=FAILED/SUCCESS

fix: #731
Change-Id: I2b572f35faffd27680e04dfa28c4696318765c77

[codecov]

Codecov Report

Merging #732 into master will increase coverage by 0.58%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master    #732      +/-   ##
===========================================
+ Coverage     70.72%   71.3%   +0.58%     
- Complexity     4321    4343      +22     
===========================================
  Files           281     281              
  Lines         20753   20753              
  Branches       2925    2924       -1     
===========================================
+ Hits          14677   14798     +121     
+ Misses         4627    4506     -121     
  Partials       1449    1449

Impacted Files	Coverage Δ	Complexity Δ
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	65.06% <100%> (ø)	47 <0> (-1)	⬇️
...baidu/hugegraph/backend/store/mysql/MysqlUtil.java	96.61% <0%> (+3.38%)	23% <0%> (ø)	⬇️
...aidu/hugegraph/backend/store/mysql/MysqlTable.java	81.85% <0%> (+8.54%)	72% <0%> (+10%)	⬆️
...u/hugegraph/backend/store/mysql/MysqlSessions.java	62.94% <0%> (+10.65%)	19% <0%> (+8%)	⬆️
...idu/hugegraph/backend/store/mysql/MysqlTables.java	89.01% <0%> (+14.01%)	1% <0%> (ø)	⬇️
...aidu/hugegraph/backend/store/mysql/MysqlStore.java	75.54% <0%> (+17.93%)	37% <0%> (+1%)	⬆️
...egraph/backend/store/mysql/MysqlStoreProvider.java	91.66% <0%> (+33.33%)	7% <0%> (+4%)	⬆️

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