fix: handle empty collections in download endpoint

[joaquingx]

Previously, the download endpoint would throw a 500 error when attempting to download from empty collections due to division by zero when calculating docs_limit. Now gracefully handles this case by falling back to MAX_WEB_DOWNLOAD_SIZE when the collection is empty or size estimation fails.

🤖 Generated with Claude Code

Description

Please include a summary of the changes, relevant motivation and context.

Issue

• Github Issue ID.

Checklist before requesting a review

• I have performed a self-review of my code.
• My code follows the style guidelines of this project.
• I have made corresponding changes to the documentation.
• New and existing tests pass locally with my changes.
• If this change is a core feature, I have added thorough tests.
• If this change affects or depends on the behavior of other estela repositories, I have created pull requests with the relevant changes in the affected repositories. Please, refer to our official documentation.
• I understand that my pull request may be closed if it becomes obvious or I did not perform all of the steps above.