Test reading the balance of a missing account

The system API should fallback and return zero in that case.

linera-execution/tests/service_runtime_apis.rs

@@ -85,6 +85,38 @@ async fn test_read_owner_balance_system_api(
     view.query_application(context, query, None).await.unwrap();
 }
 
+/// Tests if reading the balance of a missing account returns zero.
+#[proptest(async = "tokio")]
+async fn test_read_owner_balance_returns_zero_for_missing_accounts(missing_account: AccountOwner) {
+    let mut view = SystemExecutionState {
+        description: Some(ChainDescription::Root(0)),
+        ..SystemExecutionState::default()
+    }
+    .into_view()
+    .await;
+
+    let (application_id, application) = view.register_mock_application().await.unwrap();
+
+    application.expect_call(ExpectedCall::handle_query(
+        move |runtime, _context, _query| {
+            assert_eq!(
+                runtime.read_owner_balance(missing_account).unwrap(),
+                Amount::ZERO
+            );
+            Ok(vec![])
+        },
+    ));
+    application.expect_call(ExpectedCall::default_finalize());
+
+    let context = create_dummy_query_context();
+    let query = Query::User {
+        application_id,
+        bytes: vec![],
+    };
+
+    view.query_application(context, query, None).await.unwrap();
+}
+
 /// Tests the contract system API to read all account balances.
 #[proptest(async = "tokio")]
 async fn test_read_owner_balances_system_api(