Map should fill null values only when set WriteMapNullValue

[jasonjoo2010]

make it more compatible to previous versions.

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[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[codecov-io]

Codecov Report

Merging #1837 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master    #1837      +/-   ##
============================================
- Coverage     81.06%   81.05%   -0.01%     
+ Complexity     6210     6209       -1     
============================================
  Files           137      137              
  Lines         23081    23081              
  Branches       5343     5343              
============================================
- Hits          18711    18709       -2     
- Misses         3024     3025       +1     
- Partials       1346     1347       +1

Impacted Files	Coverage Δ	Complexity Δ
...m/alibaba/fastjson/serializer/FieldSerializer.java	85.71% <100%> (ø)	57 <0> (ø)	⬇️
...alibaba/fastjson/serializer/SerializerFeature.java	92.72% <100%> (ø)	9 <0> (ø)	⬇️
...baba/fastjson/serializer/ASMSerializerFactory.java	97.7% <100%> (ø)	211 <0> (ø)	⬇️
...com/alibaba/fastjson/serializer/MapSerializer.java	75% <100%> (ø)	48 <0> (ø)	⬇️
...com/alibaba/fastjson/parser/JSONReaderScanner.java	72.61% <0%> (-1.28%)	44% <0%> (-1%)

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