fix and test added for the bug

Author: Fred1155

core/sdk-core/src/main/java/software/amazon/awssdk/core/internal/async/ByteBuffersAsyncRequestBody.java

@@ -87,6 +87,9 @@ public void request(long n) {
                             int i = index.getAndIncrement();
 
                             if (i >= buffers.length) {
+                                if (completed.compareAndSet(false, true)) {
+                                    s.onComplete();
+                                }
                                 return;
                             }
 

core/sdk-core/src/test/java/software/amazon/awssdk/core/internal/async/ByteBuffersAsyncRequestBodyTest.java

@@ -81,6 +81,16 @@ public void subscriberIsMarkedAsCompleted() {
         assertEquals(1, subscriber.publishedResults.size());
     }
 
+    @Test
+    public void subscriberIsMarkedAsCompletedWhenNoBufferIsGiven() {
+        AsyncRequestBody requestBody = ByteBuffersAsyncRequestBody.of();
+        TestSubscriber subscriber = new TestSubscriber();
+
+        requestBody.subscribe(subscriber);
+        subscriber.request(1);
+        assertTrue(subscriber.onCompleteCalled);
+    }
+
     @Test
     public void subscriberIsMarkedAsCompletedWhenARequestIsMadeForMoreBuffersThanAreAvailable() {
         AsyncRequestBody requestBody = ByteBuffersAsyncRequestBody.from("Hello World!".getBytes(StandardCharsets.UTF_8));