Fix raster-dem glitches for "no content" tiles (status 204)

[ahaensler]

Raster-DEM sources like Martin return 204 when there is no content. Maplibre stores such tiles as 1x1 images, introduced in PR #3428 .

This creates glitches while rendering. Elevation looks random and there are backfill errors.

My PR skips 1x1 images and throws 404 instead. This follows the legacy code path for no content tiles.

Launch Checklist

• 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.
• Include before/after visuals or gifs if this PR includes visual changes.
• Write tests for all new functionality.
• Document any changes to public APIs.
• Post benchmark scores.
• Add an entry to CHANGELOG.md under the ## main section.

Before

After

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 91.94%. Comparing base (3f28408) to head (8bf73ac).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/source/raster_dem_tile_worker_source.ts	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5392      +/-   ##
==========================================
- Coverage   91.95%   91.94%   -0.01%     
==========================================
  Files         282      282              
  Lines       38908    38912       +4     
  Branches     6820     6827       +7     
==========================================
+ Hits        35777    35779       +2     
- Misses       3003     3005       +2     
  Partials      128      128              

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[HarelM]

I think a test is needed here.
@xabbu42 @petrsloup any insights on this?