Allow zero byte reads on raw HTTP/1.1 response streams

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/HttpConnection.cs

@@ -1747,7 +1747,7 @@ private ValueTask<int> ReadBufferedAsync(Memory<byte> destination)
             // If the caller provided buffer, and thus the amount of data desired to be read,
             // is larger than the internal buffer, there's no point going through the internal
             // buffer, so just do an unbuffered read.
-            return destination.Length >= _readBuffer.Length ?
+            return destination.Length == 0 || destination.Length >= _readBuffer.Length ?
                 ReadAsync(destination) :
                 ReadBufferedAsyncCore(destination);
         }

src/libraries/System.Net.Http/src/System/Net/Http/SocketsHttpHandler/RawConnectionStream.cs

@@ -45,9 +45,9 @@ public override async ValueTask<int> ReadAsync(Memory<byte> buffer, Cancellation
                 CancellationHelper.ThrowIfCancellationRequested(cancellationToken);
 
                 HttpConnection? connection = _connection;
-                if (connection == null || buffer.Length == 0)
+                if (connection == null)
                 {
-                    // Response body fully consumed or the caller didn't ask for any data
+                    // Response body fully consumed
                     return 0;
                 }
 
@@ -74,7 +74,7 @@ public override async ValueTask<int> ReadAsync(Memory<byte> buffer, Cancellation
                     }
                 }
 
-                if (bytesRead == 0)
+                if (bytesRead == 0 && buffer.Length != 0)
                 {
                     // A cancellation request may have caused the EOF.
                     CancellationHelper.ThrowIfCancellationRequested(cancellationToken);