fix: guard label/text normalizers against None node labels (#1194)

[freiit]

What & why

Graph nodes can carry label: None — OpenAI-compatible LLM backends occasionally emit a null label during semantic extraction. The callers fetch the label with node.get("label", node.get("id", "")) / node.get("label", ""), but dict.get(key, default) returns the default only when the key is absent; for an explicit "label": None it returns None. That None flows into helpers that call unicodedata.normalize(...), crashing the entire extract pipeline with:

TypeError: normalize() argument 2 must be str, not None

at whichever normalizer runs first (dedup → build → export). The four affected helpers:

• dedup._norm
• build._norm_label
• export._strip_diacritics
• serve._strip_diacritics

Because semantic extraction results are cached before the build step, the crash recurs on every subsequent extract until the cache is wiped, and there's no --no-dedup escape hatch. Same bug class as #454 (sanitize_label crashing on a None source_file).

The other unicodedata.normalize call sites (extract._make_id, build._normalize_id, symbol_resolution._bash_make_id, mcp_ingest) build their input via "_".join(p for p in parts if p), so they're always str — not affected.

Fix

Coerce non-str input to "" at each chokepoint (and widen the type hint to str | None). A null/empty label then normalizes to "", which the surrounding if key: guards already skip — so the offending node simply isn't considered for merging (it stays in the graph) instead of aborting the run.

Verification

Reproduced on a 6,253-file Markdown corpus via a vLLM / gpt-oss-120b OpenAI-compatible backend: every extract crashed — first at dedup._norm, then (after guarding that) at export._strip_diacritics. With all four guarded, the same corpus builds cleanly:

[graphify extract] wrote /data/graphify-out/graph.json: 16715 nodes, 16136 edges, 4272 communities
[graphify] Done

py_compile clean. Unit check: _norm(None) == "", _norm_label(None) == "", both _strip_diacritics(None) == ""; normal strings unchanged.

Fixes #1194