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 :warning:

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/top-think/think-orm/pull/527/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=top-think) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/top-think/think-orm/pull/527/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=top-think) | `12.28% <88.88%> (+0.03%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=top-think#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Hhh0121]

@liu21st 这个有问题吗，怎么关闭了