fix allowed append vl pk or el sk as nullable props by zhoney · Pull Request #1406 · apache/hugegraph

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