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

@@          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	Coverage Δ
.../org/nebula/contrib/ngbatis/proxy/MapperProxy.java	0.00% <0.00%> (ø)

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