[backport] pd: fix behavior of simulation RPC (#4733)

## Describe your changes
Backports the RPC fix in
#4731 to the
`release/v0.79.x` branch, in preparation for `v0.79.2`.

## Issue ticket number and link
Only #4731

## Checklist before requesting a review

- [x] If this code contains consensus-breaking changes, I have added the
"consensus-breaking" label. Otherwise, I declare my belief that there
are not consensus-breaking changes, for the following reason:

> RPC-only changes, already reviewed in #4731, and should be exactly the
same here.

Co-authored-by: Henry de Valence <[email protected]>

crates/core/component/dex/src/component/rpc.rs

@@ -645,7 +645,8 @@ impl SimulationService for Server {
 
         let mut state_tx = Arc::new(StateDelta::new(state));
         let execution_circuit_breaker = ExecutionCircuitBreaker::new(execution_budget);
-        let Some(swap_execution) = state_tx
+
+        let swap_execution = match state_tx
             .route_and_fill(
                 input.asset_id,
                 output_id,
@@ -655,10 +656,19 @@ impl SimulationService for Server {
             )
             .await
             .map_err(|e| tonic::Status::internal(format!("error simulating trade: {:#}", e)))?
-        else {
-            return Err(tonic::Status::failed_precondition(
-                "there are no orders to fulfill this swap",
-            ));
+        {
+            Some(swap_execution) => swap_execution,
+            None => SwapExecution {
+                traces: vec![],
+                input: Value {
+                    amount: 0u64.into(),
+                    asset_id: input.asset_id,
+                },
+                output: Value {
+                    amount: 0u64.into(),
+                    asset_id: output_id,
+                },
+            },
         };
 
         let unfilled = Value {