Reduce allocations for empty enumerators

Author: hazzik

src/NHibernate/Engine/Query/HQLQueryPlan.cs

@@ -247,7 +247,7 @@ public IAsyncEnumerable<T> PerformAsyncIterate<T>(QueryParameters queryParameter
 
 			if (Translators.Length == 0)
 			{
-				return new CollectionHelper.EmptyAsyncEnumerableClass<T>();
+				return new CollectionHelper.EmptyAsyncEnumerable<T>();
 			}
 
 			if (Translators.Length == 1)

src/NHibernate/Util/CollectionHelper.cs

@@ -43,22 +43,6 @@ public bool MoveNext()
 			}
 		}
 
-		[Serializable]
-		private class EmtpyAsyncEnumerator<T> : IAsyncEnumerator<T>
-		{
-			public T Current => throw new InvalidOperationException("EmtpyAsyncEnumerator.get_Current");
-
-			public ValueTask DisposeAsync()
-			{
-				return default;
-			}
-
-			public ValueTask<bool> MoveNextAsync()
-			{
-				return new ValueTask<bool>(false);
-			}
-		}
-
 		private class EmptyEnumerableClass : IEnumerable
 		{
 			public IEnumerator GetEnumerator()
@@ -67,14 +51,6 @@ public IEnumerator GetEnumerator()
 			}
 		}
 
-		private class EmptyAsyncEnumerableClass : IAsyncEnumerable<object>
-		{
-			public IAsyncEnumerator<object> GetAsyncEnumerator(CancellationToken cancellationToken = default)
-			{
-				return new EmtpyAsyncEnumerator<object>();
-			}
-		}
-
 		/// <summary>
 		/// A read-only dictionary that is always empty and permits lookup by <see langword="null" /> key.
 		/// </summary>
@@ -240,7 +216,6 @@ public IEnumerator GetEnumerator()
 		}
 
 		public static readonly IEnumerable EmptyEnumerable = new EmptyEnumerableClass();
-		public static readonly IAsyncEnumerable<object> EmptyAsyncEnumerable = new EmptyAsyncEnumerableClass();
 		public static readonly IDictionary EmptyMap = new EmptyMapClass();
 
 		public static IDictionary<TKey, TValue> EmptyDictionary<TKey, TValue>()
@@ -454,11 +429,23 @@ public IEnumerator GetEnumerator()
 		}
 
 		[Serializable]
-		internal class EmptyAsyncEnumerableClass<T> : IAsyncEnumerable<T>
+		internal class EmptyAsyncEnumerable<T> : IAsyncEnumerable<T>, IAsyncEnumerator<T>
 		{
+			public T Current => throw new InvalidOperationException("EmtpyAsyncEnumerator.get_Current");
+
+			public ValueTask DisposeAsync()
+			{
+				return default;
+			}
+
+			public ValueTask<bool> MoveNextAsync()
+			{
+				return new ValueTask<bool>(false);
+			}
+
 			public IAsyncEnumerator<T> GetAsyncEnumerator(CancellationToken cancellationToken = default)
 			{
-				return new EmtpyAsyncEnumerator<T>();
+				return this;
 			}
 		}