Bugfix/encoder row writer

[MMyheart]

Scenario:
1、col.isSetNullable() is true when field is set to not null, then encoding result can't be decoded correctly.
2、An out of bound exception occurs on strList when a string field has a null value

[codecov-commenter]

Codecov Report

Merging #366 (f4db29b) into master (597c0ec) will increase coverage by 0.26%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master     #366      +/-   ##
============================================
+ Coverage     62.29%   62.55%   +0.26%     
- Complexity      669      674       +5     
============================================
  Files            65       65              
  Lines          3212     3213       +1     
  Branches        453      454       +1     
============================================
+ Hits           2001     2010       +9     
+ Misses          891      885       -6     
+ Partials        320      318       -2     

Impacted Files	Coverage Δ
...ava/com/vesoft/nebula/encoder/NebulaCodecImpl.java	87.14% <0.00%> (-1.43%)	⬇️
.../java/com/vesoft/nebula/encoder/RowWriterImpl.java	39.30% <0.00%> (-0.09%)	⬇️
.../client/storage/scan/ScanVertexResultIterator.java	56.60% <0.00%> (-1.89%)	⬇️
...t/nebula/client/storage/scan/ScanVertexResult.java	60.31% <0.00%> (-1.59%)	⬇️
...la/client/storage/scan/ScanEdgeResultIterator.java	58.87% <0.00%> (+1.86%)	⬆️
...oft/nebula/client/storage/scan/ScanEdgeResult.java	69.81% <0.00%> (+1.88%)	⬆️
...java/com/vesoft/nebula/client/meta/MetaClient.java	57.06% <0.00%> (+5.43%)	⬆️

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[Nicole00]

It's a great catch, thanks for your contribution.