修正查询缓存

[Hhh0121]

1：查询不存在的数据，首次查询返回null，后续返回false


2：查询不存在的json字段，首次查询返回null，后续报错

修正：1：cache缓存方法，应当不缓存数据为空的情况，已在value和column查询修正；2：当cacheAlways方法允许缓存空数据的情况，value方法应当缓存默认值，而不是false，会导致json数据解析报错 #468 #342 @liu21st

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 12.28%. Comparing base (e35f497) to head (17e6700).

Files	Patch %	Lines
src/db/PDOConnection.php	88.88%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff              @@
##                3.0     #527      +/-   ##
============================================
+ Coverage     12.24%   12.28%   +0.03%     
- Complexity     2965     2969       +4     
============================================
  Files            64       64              
  Lines          7129     7131       +2     
============================================
+ Hits            873      876       +3     
+ Misses         6256     6255       -1     

Flag	Coverage Δ
unittests	12.28% <88.88%> (+0.03%)	⬆️

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