[codex] fix graph route ticker fallback

[sarthakagrawal927]

What changed

• Fixed the graph route so a ticker filter that matches no rows returns None instead of silently falling back to unrelated entities.
• Added a regression test for the unmatched-ticker case.

Why

• The previous ... or rows fallback ignored the caller's ticker constraint when the filtered set was empty.
• That could produce theme summaries for the wrong company instead of correctly backing off.

Validation

• uv run pytest -q tests/test_graph_route.py
• uv run ruff check src/kb/query/graph_route.py tests/test_graph_route.py