支持 json 类型值的 null，not null判断

[YepYuYu]

• 当前的判断，仅对json元素是否存在做了判断，没有对json中的值进行判断

如：

{
    "name":null,
    "test":123
}

通过whereNull("fields->name")是无法查询出来的

[codecov-commenter]

Codecov Report

Patch coverage: 57.14% and project coverage change: +0.45 🎉

Comparison is base (87911c8) 11.82% compared to head (ca4489f) 12.28%.

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Additional details and impacted files

@@             Coverage Diff              @@
##                3.0     #479      +/-   ##
============================================
+ Coverage     11.82%   12.28%   +0.45%     
- Complexity     2956     2960       +4     
============================================
  Files            64       64              
  Lines          7118     7125       +7     
============================================
+ Hits            842      875      +33     
+ Misses         6276     6250      -26     

Flag	Coverage Δ
unittests	12.28% <57.14%> (+0.45%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/db/builder/Mysql.php	49.24% <57.14%> (+1.32%)	⬆️

... and 5 files with indirect coverage changes

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