fix allowed append vl pk or el sk as nullable props

[zhoney]

Change-Id: I9514e088718414809f9b2e1477f358648a581df4

[codecov]

Codecov Report

Merging #1406 (bf81b56) into master (f57ad0d) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1406      +/-   ##
============================================
+ Coverage     61.89%   62.07%   +0.18%     
- Complexity     5811     5840      +29     
============================================
  Files           386      386              
  Lines         32192    32198       +6     
  Branches       4497     4499       +2     
============================================
+ Hits          19924    19986      +62     
+ Misses        10230    10180      -50     
+ Partials       2038     2032       -6     

Impacted Files	Coverage Δ	Complexity Δ
...idu/hugegraph/schema/builder/EdgeLabelBuilder.java	83.62% <100.00%> (+0.17%)	84.00 <0.00> (+1.00)
...u/hugegraph/schema/builder/VertexLabelBuilder.java	85.44% <100.00%> (+0.16%)	87.00 <0.00> (+1.00)
...a/com/baidu/hugegraph/backend/query/Condition.java	78.10% <0.00%> (-0.37%)	32.00% <0.00%> (ø%)
...graph/backend/store/cassandra/CassandraTables.java	88.17% <0.00%> (+0.26%)	3.00% <0.00%> (ø%)
...egraph/backend/store/cassandra/CassandraStore.java	69.71% <0.00%> (+0.35%)	54.00% <0.00%> (+1.00%)
.../baidu/hugegraph/backend/query/ConditionQuery.java	86.40% <0.00%> (+0.40%)	106.00% <0.00%> (+1.00%)
.../hugegraph/backend/serializer/TableSerializer.java	86.03% <0.00%> (+0.55%)	61.00% <0.00%> (+1.00%)
...m/baidu/hugegraph/backend/tx/GraphTransaction.java	79.95% <0.00%> (+1.04%)	306.00% <0.00%> (+5.00%)
...ain/java/com/baidu/hugegraph/task/TaskManager.java	72.59% <0.00%> (+1.48%)	26.00% <0.00%> (ø%)
...java/com/baidu/hugegraph/type/define/DataType.java	86.40% <0.00%> (+1.94%)	53.00% <0.00%> (+2.00%)
... and 7 more

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