Revert "React to runtime/594 Encoder.Convert change (#17747)"

This reverts commit ad8ecf9.

Author: dougbu

src/Http/Http.Abstractions/src/Extensions/HttpResponseWritingExtensions.cs

@@ -133,8 +133,7 @@ private static void WriteMultiSegmentEncoded(PipeWriter writer, string text, Enc
             // Therefore, we check encodedLength - totalBytesUsed too.
             while (!completed || encodedLength - totalBytesUsed != 0)
             {
-                // 'text' is a complete string, the converter should always flush its buffer.
-                encoder.Convert(source, destination, flush: true, out var charsUsed, out var bytesUsed, out completed);
+                encoder.Convert(source, destination, flush: source.Length == 0, out var charsUsed, out var bytesUsed, out completed);
                 totalBytesUsed += bytesUsed;
 
                 writer.Advance(bytesUsed);