fix bugs: schema existence check

[javeme]

fix #870

[codecov]

Codecov Report

Merging #1045 into master will increase coverage by 0.57%.
The diff coverage is 58.33%.

@@             Coverage Diff              @@
##             master    #1045      +/-   ##
============================================
+ Coverage     68.58%   69.15%   +0.57%     
- Complexity     5401     5435      +34     
============================================
  Files           329      329              
  Lines         26394    26394              
  Branches       3761     3763       +2     
============================================
+ Hits          18102    18254     +152     
+ Misses         6512     6360     -152     
  Partials       1780     1780              

Impacted Files	Coverage Δ	Complexity Δ
...du/hugegraph/schema/builder/IndexLabelBuilder.java	89.90% <50.00%> (+1.57%)	114.00 <7.00> (+4.00)
...idu/hugegraph/schema/builder/EdgeLabelBuilder.java	83.45% <60.00%> (+0.35%)	83.00 <5.00> (+4.00)
...raph/backend/store/rocksdb/RocksDBStdSessions.java	70.02% <100.00%> (ø)	30.00 <0.00> (ø)
...om/baidu/hugegraph/task/StandardTaskScheduler.java	85.26% <0.00%> (-0.36%)	67.00% <0.00%> (ø%)
...c/main/java/com/baidu/hugegraph/task/HugeTask.java	69.76% <0.00%> (-0.34%)	69.00% <0.00%> (-1.00%)
...du/hugegraph/backend/tx/GraphIndexTransaction.java	79.73% <0.00%> (-0.12%)	192.00% <0.00%> (ø%)
.../baidu/hugegraph/backend/query/ConditionQuery.java	86.20% <0.00%> (+0.43%)	101.00% <0.00%> (+1.00%)
.../backend/store/cassandra/CassandraSessionPool.java	57.14% <0.00%> (+1.02%)	11.00% <0.00%> (ø%)
...u/hugegraph/schema/builder/VertexLabelBuilder.java	85.28% <0.00%> (+2.26%)	86.00% <0.00%> (+3.00%)
... and 4 more

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