fixed bug related to runtimes in simulation mode

Author: MiloLurati

kernel_tuner/file_utils.py

@@ -105,6 +105,8 @@ def store_output_file(output_filename: str, results, tune_params, objective="tim
         timings["validation"] = result["verification_time"]
         if "times" in result:
             timings["runtimes"] = result["times"]
+        else:
+            timings["runtimes"] = []
         out["times"] = timings
 
         # encode the validity of the configuration