fix: support methods in mapper tags to set space to null.

[CorvusYe]

  <mapper namespace="...">
    <create id="createSpace" space="null">
      create space new_space ( vid_type  = INT64 );
    </create>
  </mapper>

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (983ddcc) 0.00% compared to head (985a8de) 0.00%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #254 +/- ## ====================================== Coverage 0.00% 0.00% ====================================== Files 75 75 Lines 2563 2566 +3 Branches 276 278 +2 ====================================== - Misses 2563 2566 +3 ``` | [Files](https://app.codecov.io/gh/nebula-contrib/ngbatis/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [.../org/nebula/contrib/ngbatis/proxy/MapperProxy.java](https://app.codecov.io/gh/nebula-contrib/ngbatis/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL21haW4vamF2YS9vcmcvbmVidWxhL2NvbnRyaWIvbmdiYXRpcy9wcm94eS9NYXBwZXJQcm94eS5qYXZh) | `0.00% <0.00%> (ø)` | |

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