fix union bug

[nevermore3]

What type of PR is this?

• bug
• feature
• enhancement

What problem(s) does this PR solve?

Issue(s) number:

close https://github.com/vesoft-inc/nebula-ent/issues/2992

Description:

How do you solve it?

Special notes for your reviewer, ex. impact of this fix, design document, etc:

Checklist:

Tests:

• Unit test(positive and negative cases)
• Function test
• Performance test
• N/A

Affects:

• Documentation affected (Please add the label if documentation needs to be modified.)
• Incompatibility (If it breaks the compatibility, please describe it and add the label.）
• If it's needed to cherry-pick (If cherry-pick to some branches is required, please label the destination version(s).)
• Performance impacted: Consumes more CPU/Memory

Release notes:

Please confirm whether to be reflected in release notes and how to describe:

ex. Fixed the bug .....

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +1.19% 🎉

Comparison is base (d0086dd) 77.06% compared to head (d590a1c) 78.25%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5674      +/-   ##
==========================================
+ Coverage   77.06%   78.25%   +1.19%     
==========================================
  Files        1133     1133              
  Lines       85769    85778       +9     
==========================================
+ Hits        66095    67126    +1031     
+ Misses      19674    18652    -1022     

Files Changed	Coverage Δ
src/graph/validator/SetValidator.h	100.00% <ø> (ø)
src/graph/validator/SetValidator.cpp	96.55% <100.00%> (+0.80%)	⬆️

... and 95 files with indirect coverage changes

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