Fix a compilation error

Thanks to @jknair

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

@@ -202,7 +202,7 @@ public class HttpObjectAggregator
         public HttpHeaders trailingHeaders() {
             HttpHeaders trailingHeaders = this.trailingHeaders;
             if (trailingHeaders == null) {
-                return HttpHeaders.EMPTY_HEADERS;
+                return EmptyHttpHeaders.INSTANCE;
             } else {
                 return trailingHeaders;
             }