fix: return Error::unsupported instead of panic in Scalar::to_array(MapType)

[zachschuermann]

title. fixing a panic: return error instead.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.37%. Comparing base (74aab72) to head (1907e3f).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
kernel/src/engine/arrow_expression/mod.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
- Coverage   84.38%   84.37%   -0.01%     
==========================================
  Files          80       80              
  Lines       19219    19221       +2     
  Branches    19219    19221       +2     
==========================================
  Hits        16218    16218              
- Misses       2199     2201       +2     
  Partials      802      802              

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[scovich]

Good catch. I wonder how many other random panics are lurking...