Validate empty task list in DSL builder (#1594)

Throw IllegalStateException when tasks() is called but produces no
tasks, catching the issue at build time instead of a runtime
NoSuchElementException in TaskExecutorHelper.

Signed-off-by: Matheus Cruz <matheuscruz.dev@gmail.com>

Author: mcruzdev

fluent/spec/src/main/java/io/serverlessworkflow/fluent/spec/BaseWorkflowBuilder.java

@@ -136,7 +136,11 @@ private SELF appendDo(Consumer<DBuilder> configurer) {
     configurer.accept(doBuilder);
 
     final List<TaskItem> newItems = doBuilder.build().getDo();
-    if (newItems == null || newItems.isEmpty()) return self();
+    if (newItems == null || newItems.isEmpty()) {
+      throw new IllegalStateException(
+          "Task list must contain at least one task. "
+              + "Use .tasks(d -> d.set(...)) or similar to define tasks.");
+    }
 
     final List<TaskItem> merged =
         new ArrayList<>(this.workflow.getDo() != null ? this.workflow.getDo() : List.of());

fluent/spec/src/test/java/io/serverlessworkflow/fluent/spec/WorkflowBuilderTest.java

@@ -33,6 +33,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import io.serverlessworkflow.api.types.AuthenticationPolicyUnion;
@@ -112,6 +113,17 @@ void testUseAuthenticationsBasic() {
     assertNotNull(union.getBasicAuthenticationPolicy(), "BasicAuthenticationPolicy should be set");
   }
 
+  @Test
+  void testEmptyTasksThrows() {
+    assertThrows(IllegalStateException.class, () -> WorkflowBuilder.workflow().tasks().build());
+  }
+
+  @Test
+  void testEmptyTasksConsumerThrows() {
+    assertThrows(
+        IllegalStateException.class, () -> WorkflowBuilder.workflow().tasks(d -> {}).build());
+  }
+
   @Test
   void testDoTaskSetAndForEach() {
     Workflow wf =