fix distributed evaluation on empty task shards (#1233)

* fix(evaluator): keep distributed runs alive on empty task shards

Inject a padding request when a rank receives zero docs and align request/filter synchronization across ranks so TP+DP jobs with limit<=world_size no longer crash or hang.

* style: auto-fix lint (black + isort)

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: Luodian