fix：修复json字段写入字符串null的bug

[liuqiandev]

mysql中json字段默认值为null的情况下，设置json字段会出现写入字符串null的情况

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7b0b8ea) 12.24% compared to head (1db43d4) 12.24%.

Files	Patch %	Lines
src/db/Builder.php	33.33%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                3.0     #516      +/-   ##
============================================
- Coverage     12.24%   12.24%   -0.01%     
  Complexity     2965     2965              
============================================
  Files            64       64              
  Lines          7128     7129       +1     
============================================
  Hits            873      873              
- Misses         6255     6256       +1     

Flag	Coverage Δ
unittests	12.24% <33.33%> (-0.01%)	⬇️

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