fix!: mark return type for custom fields as optional

[LiamMartens]

This would be better from a type safety perspective since Sanity doesn't strictly require data to be available.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage 👍

Coverage data is based on head (ea92108) compared to base (f9f4bba).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #294   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines         1391      1394    +3     
  Branches        78        78           
=========================================
+ Hits          1391      1394    +3     

Impacted Files	Coverage Δ
src/extractor.ts	100.00% <100.00%> (ø)
src/query/builder.ts	100.00% <100.00%> (ø)
src/schema/fields.ts	100.00% <100.00%> (ø)

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[LiamMartens]

@danielroe if we get this through, what would you want to do from a version perspective? Since this will suddenly introduce a lot of type errors if they weren't checked properly before in people's codebases. Might be considered breaking.

[danielroe]

I agree. This is a right but breaking change.

Unrelated, but any chance you could upgrade your pnpm version or revert lockfile changes?