[release-20.0] Fix evalengine crashes on unexpected types (#18254)

[vitess-bot]

Description

This is a backport of #18254

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 84.00000% with 8 lines in your changes missing coverage. Please review.

Project coverage is 66.41%. Comparing base (4bd42e8) to head (c88f32c).
Report is 1 commits behind head on release-20.0.

Files with missing lines	Patch %	Lines
go/vt/vtgate/evalengine/fn_compare.go	83.67%	8 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           release-20.0   #18256      +/-   ##
================================================
+ Coverage         66.40%   66.41%   +0.01%     
================================================
  Files              1543     1543              
  Lines            244895   244938      +43     
================================================
+ Hits             162624   162683      +59     
+ Misses            82271    82255      -16     

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