fix(validator): don't reject merged PRs with a missing base_ref

[jaytbarimbao-collab]

Summary

check_merged_branch_eligibility gated the base_ref check with not branch_matches_pattern(base_ref or '', acceptable). When the mirror omits base_ref on pre-backfill rows (base_ref is Optional[str] → None), base_ref or '' becomes '', which never matches the acceptable branch set — so a valid MERGED PR is dropped at load time and never enters merged_prs, earning no OSS contribution score even though it still counts toward eligibility.

The fix gates only when base_ref is present, so a missing base_ref falls through rather than false-positive-blocking. This matches three existing precedents:

• the head_ref check in the same function (if acceptable and head_ref and ...), which already skips on a falsy head_ref;
• the function's own docstring: "missing head_ref/head_repo_full_name skips the head_ref check rather than false-positive-blocking on older data";
• issue discovery, which calls this same shared gate only when base_ref is not None (issue_discovery/scan.py), covered by test_missing_base_ref_falls_through_to_solved.

Related Issues

Fixes #1599

Type of Change

• Bug fix

Testing

• Tests added — TestEligibilityGate::test_missing_base_ref_falls_through_not_blocked asserts a merged PR with base_ref=None falls through the gate. Fails on test before the fix (blocked), passes after. A real non-matching base_ref is still blocked (existing eligibility tests unchanged, all 21 green).
• Full suite green (961 passed), ruff + pyright clean.

Checklist

• Code follows project style guidelines
• Self-review completed
• Changes are documented (if applicable)

[jaytbarimbao-collab]

@anderdc @LandynDev fixes #1599 (bug, filed by @luciferlive112116). The shared check_merged_branch_eligibility gate false-positive-blocks a MERGED PR when the mirror omits base_ref on pre-backfill rows — the fix gates only when base_ref is present, matching the head_ref check in the same function, the docstring's stated intent, and issue discovery's base_ref is not None guard (which has test_missing_base_ref_falls_through_to_solved). +regression test. Couldn't self-apply the bug label (no triage access).