tools: Handle empty list in cleanup-excludelists.sh

[anomiex]

Proposed changes

If an exclude list is empty, the script would lint everything (and try to add anything failing to the list) instead of linting nothing. Add checks for this situation.

Related product discussion/links

#50989 (review)

Does this pull request change what data or activity we track or use?

No

Testing instructions

• Check that the script didn't run eslint over the whole monorepo.

[github-actions]

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