Coerce empty-string cause category to None (0068)

`_build_cause_chain` recorded an empty-string `category` verbatim, but
`CauseLink.category` is documented as a non-empty string or None and
`_derive_cause` already treats an empty string as no-category. Coerce it
to None so the chain representation matches. No exception carries an
empty-string category in practice; addresses PR review feedback.

Author: chris-colinsky

src/openarmature/graph/middleware/failure_isolation.py

@@ -87,7 +87,7 @@ def _build_cause_chain(exc: Exception) -> tuple[CauseLink, ...]:
         category = getattr(current, "category", None)
         links.append(
             CauseLink(
-                category=category if isinstance(category, str) else None,
+                category=category if isinstance(category, str) and category else None,
                 message=str(current),
                 carrier=isinstance(current, NodeException),
             )