fix: guard against empty query in SPARQL generator (#934)

Split the query once and check the parts list before indexing,
preventing an IndexError if the LLM returns an empty or
whitespace-only string.

Fixes #870.

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

@@ -202,11 +202,14 @@ ASK {{
 
             if response and isinstance(response, dict):
                 query = response.get('query', '').strip()
-                if query.upper().startswith(('SELECT', 'ASK', 'CONSTRUCT', 'DESCRIBE')):
+                parts = query.split()
+                if parts and parts[0].upper() in (
+                    'SELECT', 'ASK', 'CONSTRUCT', 'DESCRIBE',
+                ):
                     return SPARQLQuery(
                         query=query,
                         variables=self._extract_variables(query),
-                        query_type=query.split()[0].upper(),
+                        query_type=parts[0].upper(),
                         explanation=response.get('explanation', 'Generated by LLM'),
                         complexity_score=self._calculate_complexity(query)
                     )