fix(validator): fall through OSS scoring when mirror omits base_ref

[RealDiligent]

Fixes #1636

Summary

Pre-backfill mirror rows can omit base_ref. OSS scoring treated null as '' and rejected those merged PRs at load time, while issue discovery already fall-throughs missing branch metadata.

Skip the base-ref gate when base_ref is None, matching the existing null head_ref / head_repo_full_name behavior and issue-discovery parity.

Root cause

check_merged_branch_eligibility used base_ref or '', so a missing base_ref became an empty string that never matched the acceptable branch set. Valid merged PRs were dropped before scoring.

Fix approach

Gate on base_ref only when it is present (base_ref is not None), consistent with issue discovery (scan.py) and existing null head_ref fall-through.

Impact

Pre-backfill mirror rows and any merged PR missing branch metadata can score OSS contributions instead of being silently dropped.

Risk / tradeoffs

• Intentionally permissive for absent metadata only; explicit non-matching base_ref values are still rejected.
• No producer-side mirror change required.

Test plan

• Added test_null_base_ref_skips_check in tests/validator/oss_contributions/mirror/test_scoring.py
• CI validated on fork (Tests + Lint passing):
  • https://github.com/RealDiligent/gittensor/actions/runs/29393812928
  • https://github.com/RealDiligent/gittensor/actions/runs/29393813897