Handle when resp_vol4s are empty

From an evaluation standpoint, if a system returns an invalid response, the test harness should fail the evaluation rather than exit with an exception.

monitoring/uss_qualifier/scenarios/astm/utm/evaluation.py

@@ -27,6 +27,10 @@ def validate_op_intent_details(
 
     # Check that the USS is providing reasonable volume 4D
     resp_vol4s = op_intent_details.volumes + op_intent_details.off_nominal_volumes
+    if len(resp_vol4s) == 0:
+        errors_text.append('OperationalIntentResponse did not return required volumes or off nominal volumes.')
+        return "; ".join(errors_text) if len(errors_text) > 0 else None
+
     vol4c = Volume4DCollection.from_f3548v21(resp_vol4s)
     resp_alts = vol4c.meter_altitude_bounds
     resp_start = vol4c.time_start.datetime