Fix type instability in Enzyme extension

[codecov[bot]]

Codecov Report

Merging #586 (36646c4) into master (0db6844) will increase coverage by 0.14%. The diff coverage is 0.00%.

:exclamation: Current head 36646c4 differs from pull request most recent head 45bfe0c. Consider uploading reports for the commit 45bfe0c to get more accurate results

@@            Coverage Diff             @@
##           master     #586      +/-   ##
==========================================
+ Coverage   10.01%   10.16%   +0.14%     
==========================================
  Files          41       41              
  Lines        2446     2450       +4     
==========================================
+ Hits          245      249       +4     
  Misses       2201     2201              

Files Changed	Coverage Δ
ext/OptimizationEnzymeExt.jl	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

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[Vaibhavdixit02]

Thanks a lot, @wsmoses!