[Event Hubs] Avoid Possible Allocation (#22632)

The focus of these changes is to alter the approach used for copying an
AMQP message section dictionary to avoid a possible allocation of the `Keys`
collection of the source dictionary when no items are present.

Author: jsquire

sdk/eventhub/Azure.Messaging.EventHubs/src/Amqp/AmqpAnnotatedMessageExtensions.cs

@@ -399,9 +399,12 @@ private static void CopyPropertiesSection(AmqpMessageProperties source,
         private static void CopyDictionary<TKey, TValue>(IDictionary<TKey, TValue> source,
                                                          IDictionary<TKey, TValue> destination)
         {
-            foreach (var key in source.Keys)
+            if (source.Count > 0)
             {
-                destination[key] = source[key];
+                foreach (var pair in source)
+                {
+                    destination[pair.Key] = pair.Value;
+                }
             }
         }
     }