ci: skip coverage upload on missing files

[almeidx]

Please describe the changes this PR makes and why it should be merged:
The Test step only runs for the packages that were modified (either compared to the previous commit, in the case of a merge, or in the entire pull request), which means that the coverage upload fails for the other packages, where the tests weren't ran

To prevent the fails and consequently speed up the Tests workflow, skipped the coverage upload for packages that don't have the coverage result file.

discord.js/.github/workflows/tests.yml

Lines 38 to 44 in bf83db9

	- name: Tests (PR)
	if: ${{ github.event_name != 'push' }}
	run: pnpm exec turbo run test --filter="...[origin/${{ github.event_name == 'pull_request' && github.event.pull_request.base.ref || 'main' }}]" --concurrency=4
	- name: Tests (Push)
	if: ${{ github.event_name == 'push' }}
	run: pnpm exec turbo run test --filter="...[HEAD^1]" --concurrency=4

Example of upload failure:
https://github.com/discordjs/discord.js/actions/runs/10469755109/job/28993460857#step:13:64

Status and versioning classification:

[vercel]

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2 Skipped Deployments

Name	Status	Preview	Comments	Updated (UTC)
discord-js	⬜️ Ignored (Inspect)	Visit Preview		Aug 22, 2024 11:52am
discord-js-guide	⬜️ Ignored (Inspect)	Visit Preview		Aug 22, 2024 11:52am

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.90%. Comparing base (bba0e72) to head (e1515ca).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10453      +/-   ##
==========================================
- Coverage   50.91%   50.90%   -0.01%     
==========================================
  Files         228      228              
  Lines       20750    20746       -4     
  Branches     1258     1256       -2     
==========================================
- Hits        10565    10561       -4     
  Misses      10141    10141              
  Partials       44       44              

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