Check that we have demand before requesting from upstram

Author: sugmanue

client/client-http/src/main/java/software/amazon/smithy/java/client/http/HttpClientDataStream.java

@@ -121,8 +121,9 @@ private void drain() {
         private void drainAndRequest() {
             drain();
 
-            if (queue.isEmpty() && !senderFinished.get()) {
-                upstreamSubscription.request(demand.get());
+            var currentDemand = demand.get();
+            if (currentDemand > 0 && queue.isEmpty() && !senderFinished.get()) {
+                upstreamSubscription.request(currentDemand);
             }
         }
     }