[3.0] 修复 FieldTypeTransform 无法转换时允许返回 null 用于解决兼容问题

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 17.37%. Comparing base (e35f497) to head (23f01ce). Report is 70 commits behind head on 3.0.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## 3.0 #609 +/- ## ============================================ + Coverage 12.24% 17.37% +5.13% - Complexity 2965 3004 +39 ============================================ Files 64 64 Lines 7129 7199 +70 ============================================ + Hits 873 1251 +378 + Misses 6256 5948 -308 ``` | [Flag](https://app.codecov.io/gh/top-think/think-orm/pull/609/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/609/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=top-think) | `17.37% <ø> (+5.13%)` | :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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