feat: skip upload and backup for zero-byte files and update validation logic

[egalvis27]

What is Changed / Added

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• I added a centralized empty-file validation rule in the upload size validator, with an opt-in flag so existing flows keep their current behavior unless they explicitly disable empty files.
• I introduced a new EMPTY_FILE validation reason and used it in backup upload/update flows only, so zero-byte files are skipped before any content upload attempt.
• I updated backup upload logic to short-circuit cleanly for empty files, avoiding both content upload and metadata creation for new backup entries.
• I updated backup update logic to short-circuit cleanly for empty files as well, avoiding unnecessary override calls for existing entries.
• I kept max-size rejection behavior unchanged for real size-limit cases, and made sure empty-file skips do not trigger max-size rejection tracking/modal behavior.

Why

• We were seeing inconsistent failures when the client tried to upload zero-byte files during backup (for example, server 500 in some environments and Invalid size in others).
• Even though the backup could continue for other files, these attempts generated noisy error logs and, in some cases, unnecessary retry behavior.
• By handling zero-byte files earlier and consistently, we prevent avoidable upload calls, reduce retry/log noise, and make backup behavior more predictable.
• Centralizing this in the validator keeps the rule in one place and avoids duplicating ad-hoc checks across multiple upload paths.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud