Do not fail when compressing empty HttpContent (#13655)

Motivation:

HttpContentCompressor fails with an exception when trying to write an
empty HttpContent:

```
io.netty.handler.codec.EncoderException: MessageToMessageCodec$1 must produce at least one message.
	at io.netty.handler.codec.MessageToMessageEncoder.write(MessageToMessageEncoder.java:99)
	at io.netty.handler.codec.MessageToMessageCodec.write(MessageToMessageCodec.java:116)
	at io.netty.channel.AbstractChannelHandlerContext.invokeWrite0(AbstractChannelHandlerContext.java:879)
	at io.netty.channel.AbstractChannelHandlerContext.invokeWrite(AbstractChannelHandlerContext.java:863)
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:968)
	at io.netty.channel.AbstractChannelHandlerContext.write(AbstractChannelHandlerContext.java:856)
	at io.netty.channel.ChannelDuplexHandler.write(ChannelDuplexHandler.java:115)
```

Modification:

If the compression has no output, write an empty dummy HttpContent. This
is not an ideal solution, but this should not happen often anyway.

Result:

HttpContentCompressor does not fail anymore.

---------

Co-authored-by: Norman Maurer <norman_maurer@apple.com>

codec-http/src/main/java/io/netty/handler/codec/http/HttpContentEncoder.java

@@ -17,6 +17,7 @@
 
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufHolder;
+import io.netty.buffer.Unpooled;
 import io.netty.channel.ChannelHandlerContext;
 import io.netty.channel.embedded.EmbeddedChannel;
 import io.netty.handler.codec.DecoderResult;
@@ -208,6 +209,9 @@ protected void encode(ChannelHandlerContext ctx, HttpObject msg, List<Object> ou
                 ensureContent(msg);
                 if (encodeContent((HttpContent) msg, out)) {
                     state = State.AWAIT_HEADERS;
+                } else if (out.isEmpty()) {
+                    // MessageToMessageCodec needs at least one output message
+                    out.add(new DefaultHttpContent(Unpooled.EMPTY_BUFFER));
                 }
                 break;
             }

codec-http/src/test/java/io/netty/handler/codec/http/HttpContentCompressorTest.java

@@ -973,6 +973,21 @@ public void testMultipleAcceptEncodingHeaders() {
         assertTrue(ch.finishAndReleaseAll());
     }
 
+    @Test
+    public void testEmpty() {
+        EmbeddedChannel ch = new EmbeddedChannel(new HttpContentCompressor());
+        assertTrue(ch.writeInbound(newRequest()));
+
+        DefaultHttpResponse response = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
+        response.headers().add(HttpHeaderNames.CONTENT_LENGTH, 0);
+        assertTrue(ch.writeOutbound(response));
+        assertTrue(ch.writeOutbound(new DefaultHttpContent(Unpooled.EMPTY_BUFFER)));
+        assertTrue(ch.writeOutbound(DefaultLastHttpContent.EMPTY_LAST_CONTENT));
+
+        ch.checkException();
+        ch.finishAndReleaseAll();
+    }
+
     private static FullHttpRequest newRequest() {
         FullHttpRequest req = new DefaultFullHttpRequest(HttpVersion.HTTP_1_1, HttpMethod.GET, "/");
         req.headers().set(HttpHeaderNames.ACCEPT_ENCODING, "gzip");