fix: allen gene exp query w a list of genes

[AhmetNSimsek]

Test code included in changes

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 20.83333% with 19 lines in your changes are missing coverage. Please review.

Project coverage is 50.28%. Comparing base (5a0e6b3) to head (e594a7e).
Report is 437 commits behind head on main.

Files	Patch %	Lines
siibra/livequeries/allen.py	20.83%	19 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##             main     #576       +/-   ##
===========================================
+ Coverage   36.81%   50.28%   +13.46%     
===========================================
  Files          61       69        +8     
  Lines        5421     6706     +1285     
===========================================
+ Hits         1996     3372     +1376     
+ Misses       3425     3334       -91     

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