fix(tianmu): default value of the field take unaffect in load #1865

[Double0101]

Cause: in the function ParsingStrategy::ParseResult ParsingStrategy::GetOneRow field->val_str(str) cannot distinguish 0 and NULL value. Solution: Check whether field's default value is NULL.

Summary about this PR

Issue Number: close #1865

Tests Check List

• [ ] Unit test
• [x] Integration test
• [ ] Manual test (add detailed scripts or steps below)
• [ ] No code

Changelog

• [ ] New Feature
• [x] Bug Fix
• [ ] Performance Improvement
• [ ] Build/Testing/CI/CD
• [ ] Documentation
• [ ] Not for changelog (changelog entry is not required)

Documentation

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features

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[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -5.12 :warning:

Comparison is base (7d03695) 60.25% compared to head (d096d0f) 55.13%.

:exclamation: Current head d096d0f differs from pull request most recent head 52c072d. Consider uploading reports for the commit 52c072d to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## stonedb-5.7-dev #1896 +/- ## =================================================== - Coverage 60.25% 55.13% -5.12% =================================================== Files 1837 2031 +194 Lines 380678 431524 +50846 =================================================== + Hits 229371 237926 +8555 - Misses 151307 193598 +42291 ``` | [Impacted Files](https://app.codecov.io/gh/stoneatom/stonedb/pull/1896?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stoneatom) | Coverage Δ | | |---|---|---| | [storage/tianmu/loader/parsing\_strategy.cpp](https://app.codecov.io/gh/stoneatom/stonedb/pull/1896?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stoneatom#diff-c3RvcmFnZS90aWFubXUvbG9hZGVyL3BhcnNpbmdfc3RyYXRlZ3kuY3Bw) | `70.11% <100.00%> (+0.25%)` | :arrow_up: | ... and [274 files with indirect coverage changes](https://app.codecov.io/gh/stoneatom/stonedb/pull/1896/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=stoneatom)

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