Fix typing of Map.getLayer() and Style.getLayer()

[RoystonS]

Launch Checklist

Fixes #2968

• Confirm your changes do not include backports from Mapbox projects (unless with compliant license) - if you are not sure about this, please ask!
• Briefly describe the changes in this PR.
• Link to related issues. Define return type of getSource as possibly undefined #724 made the equivalent change to getSource in December 2021.
• Include before/after visuals or gifs if this PR includes visual changes. N/A
• Write tests for all new functionality. This includes typing changes only.
• Document any changes to public APIs. This change actually makes the advertised API match the documented APIs.
• Post benchmark scores. This does not affect generated code at all, so can't affect performance.
• Add an entry to CHANGELOG.md under the ## main section.

Changes

This PR changes the return types of:

• Map.getLayer() from returning StyleLayer to StyleLayer | undefined
• Style.getLayer() from returning StyleLayer to StyleLayer | undefined

Those declared return types are changing to match the existing documented behaviour, and brings them in line with the existing similar Map.getSource() and Style.getSource() methods.

Notes

• Does this constitute a (breaking) public API change?

Yes and no. It's making the TypeScript-advertised type match the already-documented and implemented behaviour. It is possible that this could cause existing code to fail to compile if it's calling getLayer() and is compiled with a strict type-checking mode that will flag up that the return type is not being checked for undefined.

See PR #724 for earlier work which made the same change to the getSource() methods.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (deb1276) 75.01% compared to head (427fbb6) 75.01%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2969   +/-   ##
=======================================
  Coverage   75.01%   75.01%           
=======================================
  Files         240      240           
  Lines       19110    19110           
  Branches     4311     4311           
=======================================
  Hits        14336    14336           
  Misses       4774     4774           

Files Changed	Coverage Δ
src/style/style.ts	83.78% <100.00%> (ø)
src/ui/map.ts	82.92% <100.00%> (ø)

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[RoystonS]

Thank you!