Add a JSONReader.Feature that can convert empty string to null

[rowstop]

What this PR does / why we need it?

Add a JSONReader.Feature that can convert empty string to null ,for issue #2317 #2060

Summary of your change

需要注意这里的实现，有些集合不支持 null，而且我觉得直接过滤 null 值也合适，我这里直接过滤了，看下这么做否合适
但是动态字节码的逻辑里并没有过滤null值，不太敢改这一块，
FieldReaderStringField

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 35.29412% with 22 lines in your changes are missing coverage. Please review.

Project coverage is 77.41%. Comparing base (6a1fd00) to head (37e74cd).
Report is 3 commits behind head on main.

Files	Patch %	Lines
...in/java/com/alibaba/fastjson2/JSONReaderJSONB.java	11.11%	10 Missing and 6 partials ⚠️
...ibaba/fastjson2/reader/FieldReaderStringField.java	0.00%	1 Missing and 2 partials ⚠️
...ain/java/com/alibaba/fastjson2/JSONReaderUTF8.java	0.00%	1 Missing and 1 partial ⚠️
...baba/fastjson2/reader/ObjectReaderImplListStr.java	80.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2321      +/-   ##
============================================
- Coverage     77.43%   77.41%   -0.02%     
- Complexity    22731    22743      +12     
============================================
  Files           634      634              
  Lines         85225    85256      +31     
  Branches      19322    19337      +15     
============================================
+ Hits          65995    66003       +8     
- Misses        11229    11243      +14     
- Partials       8001     8010       +9     

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[wenshao]

和其他JSONReader.Feature一致，EmptyStringToNull改为EmptyStringAsNull ？

[rowstop]

和其他JSONReader.Feature一致，EmptyStringToNull改为EmptyStringAsNull ？

好的 看下