fix(client): startWorkflow must not return a null workflow id

A successful response carrying no workflow id came back as a null return
value, so a caller saw a successful start with no id and no way to find the
execution. It now raises instead. Scoped to startWorkflow; deserialization is
unchanged for every other endpoint.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mp-orkes

conductor-client/src/main/java/com/netflix/conductor/client/http/WorkflowClient.java

@@ -210,10 +210,19 @@ public String startWorkflow(StartWorkflowRequest startWorkflowRequest) {
                 .build();
 
         ConductorClientResponse<String> resp = client.execute(request, STRING_TYPE);
+        String workflowId = resp.getData();
+        if (workflowId == null || workflowId.isBlank()) {
+            // Error. Indeterminate outcome.
+            throw new ConductorClientException(
+                    "Error when starting workflow. No workflow id was returned",
+                    resp.getStatusCode(),
+                    resp.getHeaders(),
+                    null);
+        }
 
         eventDispatcher
                 .publish(new WorkflowStartedEvent(startWorkflowRequest.getName(), startWorkflowRequest.getVersion()));
-        return resp.getData();
+        return workflowId;
     }
 
     public void checkAndUploadToExternalStorage(StartWorkflowRequest startWorkflowRequest) {

conductor-client/src/test/java/com/netflix/conductor/client/http/StartWorkflowEmptyResponseTest.java

@@ -0,0 +1,95 @@
+/*
+ * Copyright 2026 Conductor Authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * <p>
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+package com.netflix.conductor.client.http;
+
+import java.io.IOException;
+
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import com.netflix.conductor.client.exception.ConductorClientException;
+import com.netflix.conductor.common.metadata.workflow.RerunWorkflowRequest;
+import com.netflix.conductor.common.metadata.workflow.StartWorkflowRequest;
+
+import okhttp3.mockwebserver.MockResponse;
+import okhttp3.mockwebserver.MockWebServer;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+/**
+ * A successful response carrying no workflow id used to come back as a null return value, so a
+ * caller saw a "successful" start with no id and no error. It must raise instead.
+ */
+class StartWorkflowEmptyResponseTest {
+
+    private MockWebServer server;
+    private WorkflowClient workflowClient;
+
+    @BeforeEach
+    void setUp() throws IOException {
+        server = new MockWebServer();
+        server.start();
+        workflowClient = new WorkflowClient(new ConductorClient(server.url("/api").toString()));
+    }
+
+    @AfterEach
+    void tearDown() throws IOException {
+        server.shutdown();
+        Thread.interrupted();
+    }
+
+    private StartWorkflowRequest request() {
+        StartWorkflowRequest request = new StartWorkflowRequest();
+        request.setName("greetings");
+        request.setVersion(1);
+        return request;
+    }
+
+    @Test
+    void emptyTwoHundredBodyRaisesInsteadOfReturningNull() {
+        server.enqueue(new MockResponse().setResponseCode(200).setBody(""));
+
+        ConductorClientException e =
+                assertThrows(
+                        ConductorClientException.class,
+                        () -> workflowClient.startWorkflow(request()));
+
+        assertEquals(200, e.getStatus());
+        assertTrue(
+                e.getMessage().contains("No workflow id was returned"),
+                "message should say what went wrong, was: " + e.getMessage());
+    }
+
+    @Test
+    void aBodyIsStillReturned() {
+        server.enqueue(
+                new MockResponse()
+                        .setResponseCode(200)
+                        .setBody("3919784b-7691-11f1-a292-00163e983f22"));
+
+        assertEquals(
+                "3919784b-7691-11f1-a292-00163e983f22", workflowClient.startWorkflow(request()));
+    }
+
+    @Test
+    void otherEndpointsAreUnaffected() {
+        // The check is scoped to startWorkflow; an empty body elsewhere still deserializes to null.
+        server.enqueue(new MockResponse().setResponseCode(200).setBody(""));
+
+        assertNull(workflowClient.rerunWorkflow("wf-1", new RerunWorkflowRequest()));
+    }
+}