Allow use of a default MemoryPoolFeature on ConnectionContext (#18579)

Author: Ninds

src/Servers/Kestrel/Core/src/Middleware/Http3ConnectionMiddleware.cs

@@ -31,7 +31,7 @@ public Task OnConnectionAsync(MultiplexedConnectionContext connectionContext)
                 ConnectionContext = connectionContext,
                 ServiceContext = _serviceContext,
                 ConnectionFeatures = connectionContext.Features,
-                MemoryPool = memoryPoolFeature.MemoryPool,
+                MemoryPool = memoryPoolFeature?.MemoryPool ?? System.Buffers.MemoryPool<byte>.Shared,
                 LocalEndPoint = connectionContext.LocalEndPoint as IPEndPoint,
                 RemoteEndPoint = connectionContext.RemoteEndPoint as IPEndPoint
             };

src/Servers/Kestrel/Core/src/Middleware/HttpConnectionMiddleware.cs

@@ -33,7 +33,7 @@ public Task OnConnectionAsync(ConnectionContext connectionContext)
                 Protocols = _protocols,
                 ServiceContext = _serviceContext,
                 ConnectionFeatures = connectionContext.Features,
-                MemoryPool = memoryPoolFeature.MemoryPool,
+                MemoryPool = memoryPoolFeature?.MemoryPool ?? System.Buffers.MemoryPool<byte>.Shared,
                 Transport = connectionContext.Transport,
                 LocalEndPoint = connectionContext.LocalEndPoint as IPEndPoint,
                 RemoteEndPoint = connectionContext.RemoteEndPoint as IPEndPoint