fix(github): harden cross-reference PR parser against null GraphQL fields

[codevector96]

Summary

_search_issue_referencing_prs_graphql read the PR base repo with
pr.get('baseRepository', {}).get('nameWithOwner', ''). The {} default only
applies when the key is absent — GitHub's GraphQL API returns the key
present with an explicit null (e.g. when a referencing PR's base repository
was deleted or is inaccessible). In that case the chained .get runs on None
and raises AttributeError.

find_prs_for_issue wraps this call in except Exception -> return None
(the lookup-failure sentinel). So a single unresolvable timeline node made the
entire issue's submission lookup report as "GitHub lookup failed" instead of
skipping that one node and returning the valid PRs — the exact failure mode the
recent read-failed/empty-submissions work (#1492/#1554) was tightening.

The sibling closure-event helpers in the same file already guard these nulls
correctly (_select_current_close_event uses if node and ... and
.get('nodes', []) or []; _solver_from_closed_event uses
((closer.get('baseRepository') or {}).get('nameWithOwner') or '')). This PR
brings the cross-reference parser in line with that convention:

• null timelineItems → treated as no nodes
• null timeline node → skipped
• null baseRepository → skipped (not fatal)

Related Issues

None — surfaced while reading the issue-submission lookup path.

Type of Change

• Bug fix

Testing

Adds TestSearchIssueReferencingPRsGraphQL covering each null path directly:
a baseRepository: null PR node is skipped while valid PRs still return, a
null timeline node is skipped, and a null timelineItems payload yields []
instead of raising. Existing tests unchanged. ruff check / ruff format
clean.

Checklist

• Follows repository conventions (mirrors the null-guarding already used by the sibling closure-event helpers)
• Self-reviewed
• No unrelated files touched (1 source file + its test)