Fix null iterator deref in printCsLensTypeByMetadata

[kevinbackhouse]

Fixes: #1763

This is the same type of bug as I fixed in #1758, but it's in new code that only exists on the main branch. The new code was added in this commit.

The CodeQL query that I wrote would have found this bug too, so I am going to try adding it to our code scanning setup.

[kevinbackhouse]

I converted this to draft, because I would like to try adding the CodeQL query to our code scanning before merging this.

[codecov]

Codecov Report

Merging #1767 (033175e) into main (c7757d7) will increase coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1767      +/-   ##
==========================================
+ Coverage   67.29%   67.36%   +0.06%     
==========================================
  Files         151      151              
  Lines       20799    20802       +3     
==========================================
+ Hits        13997    14013      +16     
+ Misses       6802     6789      -13     

Impacted Files	Coverage Δ
src/canonmn_int.cpp	74.61% <50.00%> (+4.30%)	⬆️

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[kevinbackhouse]

#1768 adds the CodeQL query.

[hassec]

ohoh this was def. me who forgot those returns 🤦

Thanks for fixing this! 👍

[kevinbackhouse]

I rebased onto main to rerun the CodeQL analysis now that #1768 is merged.