fix: guard against IndexError on empty query string in SPARQL generator

When query.split() returns an empty list (which can happen if the
query is empty or whitespace-only), indexing with [0] raises an
IndexError. Extract the split result first and guard against an
empty list before accessing the first element.

Fixes #870

trustgraph-flow/trustgraph/query/ontology/sparql_generator.py

@@ -203,10 +203,12 @@ ASK {{
             if response and isinstance(response, dict):
                 query = response.get('query', '').strip()
                 if query.upper().startswith(('SELECT', 'ASK', 'CONSTRUCT', 'DESCRIBE')):
+                    parts = query.split()
+                    query_type = parts[0].upper() if parts else 'SELECT'
                     return SPARQLQuery(
                         query=query,
                         variables=self._extract_variables(query),
-                        query_type=query.split()[0].upper(),
+                        query_type=query_type,
                         explanation=response.get('explanation', 'Generated by LLM'),
                         complexity_score=self._calculate_complexity(query)
                     )