chore: replace return None by error

Author: Ludovic CHAPELET

src/pytest_bdd/parser.py

@@ -238,7 +238,7 @@ def find_scenario_example_from_context(self, context: Mapping[str, object]) -> E
             example_param = dict(zip(example.example_params, example.examples[0]))
             if example_param == context:
                 return example
-        return None
+        raise ValueError("No matching example found for the given context.")
 
 
 @dataclass(eq=False)