docs: explicitly document empty list and search responses

PiperOrigin-RevId: 439957204

google/maps/fleetengine/delivery/v1/delivery_api.proto

@@ -222,6 +222,9 @@ message ListDeliveryVehiclesRequest {
 // The `ListDeliveryVehicles` response message.
 message ListDeliveryVehiclesResponse {
   // The set of delivery vehicles that meet the requested filtering criteria.
+  // When no filter is specified, the request returns all delivery vehicles. A
+  // successful response can also be empty. An empty response indicates that no
+  // delivery vehicles were found meeting the requested filter criteria.
   repeated DeliveryVehicle delivery_vehicles = 1;
 
   // You can pass this token in the `ListDeliveryVehiclesRequest` to continue to
@@ -329,7 +332,9 @@ message SearchTasksRequest {
 // The `SearchTasks` response. It contains the set of Tasks that meet the search
 // criteria in the `SearchTasksRequest`.
 message SearchTasksResponse {
-  // The set of Tasks for the requested `tracking_id`.
+  // The set of Tasks for the requested `tracking_id`. A successful response can
+  // also be empty. An empty response indicates that no Tasks are associated
+  // with the supplied `tracking_id`.
   repeated Task tasks = 1;
 
   // Pass this token in the `SearchTasksRequest` to continue to
@@ -405,7 +410,10 @@ message ListTasksRequest {
 // The `ListTasks` response that contains the set of Tasks that meet the filter
 // criteria in the `ListTasksRequest`.
 message ListTasksResponse {
-  // The set of Tasks that meet the requested filtering criteria.
+  // The set of Tasks that meet the requested filtering criteria. When no filter
+  // is specified, the request returns all tasks. A successful response can also
+  // be empty. An empty response indicates that no Tasks were found meeting the
+  // requested filter criteria.
   repeated Task tasks = 1;
 
   // Pass this token in the `ListTasksRequest` to continue to list results.