fix hbase scan startkey is null NPE

[JackyYangPassion]

Fix bug: HBase 表预分区 分区扫描数据发生NPE

fix #1690

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #1692 (63e025c) into master (cd22604) will increase coverage by 2.24%.
The diff coverage is 75.00%.

@@             Coverage Diff              @@
##             master    #1692      +/-   ##
============================================
+ Coverage     64.84%   67.09%   +2.24%     
- Complexity     6929     7082     +153     
============================================
  Files           420      420              
  Lines         34678    34684       +6     
  Branches       4808     4810       +2     
============================================
+ Hits          22488    23272     +784     
+ Misses         9915     9079     -836     
- Partials       2275     2333      +58     

Impacted Files	Coverage Δ
...aidu/hugegraph/backend/store/hbase/HbaseTable.java	78.75% <0.00%> (+78.75%)	⬆️
...u/hugegraph/backend/store/hbase/HbaseSessions.java	58.11% <100.00%> (+58.11%)	⬆️
.../baidu/hugegraph/backend/query/ConditionQuery.java	85.94% <0.00%> (+0.26%)	⬆️
...hugegraph/backend/serializer/BinarySerializer.java	85.60% <0.00%> (+0.31%)	⬆️
...a/com/baidu/hugegraph/backend/query/Condition.java	80.14% <0.00%> (+0.72%)	⬆️
.../backend/store/cassandra/CassandraSessionPool.java	57.14% <0.00%> (+1.02%)	⬆️
...egraph/backend/store/cassandra/CassandraShard.java	52.77% <0.00%> (+1.85%)	⬆️
.../baidu/hugegraph/security/HugeSecurityManager.java	80.50% <0.00%> (+1.92%)	⬆️
...du/hugegraph/backend/store/BackendSessionPool.java	80.55% <0.00%> (+2.77%)	⬆️
.../baidu/hugegraph/backend/store/BackendSession.java	80.95% <0.00%> (+4.76%)	⬆️
... and 10 more

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[imbajin]

感谢贡献和反馈问题, 欢迎后续 hbase 相关更多的 PR 和设计提交~

[javeme]

Thank you for your contribution~ some minor comments

[JackyYangPassion]

感谢贡献和反馈问题, 欢迎后续 hbase 相关更多的 PR 和设计提交~

感谢 review ，hbase 和 tidb 相关的会持续贡献

[javeme]

LGTM