fix issue1603

[GabrielHwang]

What this PR does / why we need it?

修改 #1603 中最后位置是null时，多一个逗号的问题
BUG定位到这里：

fastjson2/core/src/main/java/com/alibaba/fastjson2/JSONWriterUTF8.java

Lines 2417 to 2423 in af0f79b

	for (Map.Entry<String, Object> entry : map.entrySet()) {
	if (!first) {
	if (off == bytes.length) {
	ensureCapacity(off + 1);
	}
	bytes[off++] = ',';
	}

Summary of your change

增加了一个是否加入逗号的判断

[wenshao]

请在本地执行mvn validate修复checkstyle的问题

[yanxutao89]

请参考其他测试用例调整。1.测试用例类名；2.采用assert验证结果。

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.02 ⚠️

Comparison is base (af0f79b) 74.70% compared to head (328e293) 74.69%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1604      +/-   ##
============================================
- Coverage     74.70%   74.69%   -0.02%     
  Complexity    20734    20734              
============================================
  Files           642      642              
  Lines         83128    83129       +1     
  Branches      18026    18027       +1     
============================================
- Hits          62100    62090      -10     
- Misses        13742    13751       +9     
- Partials       7286     7288       +2     

Impacted Files	Coverage Δ
...ain/java/com/alibaba/fastjson2/JSONWriterUTF8.java	78.03% <100.00%> (+0.07%)	⬆️

... and 10 files with indirect coverage changes

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[GabrielHwang]

请参考其他测试用例调整。1.测试用例类名；2.采用assert验证结果。
已修改，麻烦再帮忙看一下，感谢您的耐心指教 @yanxutao89

[wenshao]

https://github.com/alibaba/fastjson2/releases/tag/2.0.35
请用新版本