fix: fix fast_substitute folding array of symbolics

[AayushSabharwal]

Required for SciML/ModelingToolkit.jl#3282

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Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 6.44%. Comparing base (6202fa4) to head (1ea3df5).
Report is 40 commits behind head on master.

Files with missing lines	Patch %	Lines
src/variable.jl	0.00%	7 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##           master   #1398      +/-   ##
=========================================
+ Coverage    3.95%   6.44%   +2.48%     
=========================================
  Files          51      51              
  Lines        4824    4841      +17     
=========================================
+ Hits          191     312     +121     
+ Misses       4633    4529     -104     

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[AayushSabharwal]

The NaNMath error is trivial. The others I can't seem to replicate outside of running tests.