Avoid send empty body to ext_proc server if decodeData() never called (#28672)

* Avoid send empty body to ext_proc server if decodeData() not called

Signed-off-by: Yanjun Xiang <yanjunxiang@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 5e4f35055a30f0990430664d74f6060a2a5ff20a

envoy/extensions/filters/http/ext_proc/v3/ext_proc.proto

@@ -51,16 +51,15 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // * Whether subsequent HTTP requests are transmitted synchronously or whether they are
 //   sent asynchronously.
 // * To modify request or response trailers if they already exist
-// * To add request or response trailers where they are not present
 //
 // The filter supports up to six different processing steps. Each is represented by
 // a gRPC stream message that is sent to the external processor. For each message, the
 // processor must send a matching response.
 //
 // * Request headers: Contains the headers from the original HTTP request.
-// * Request body: Sent in a single message if the BUFFERED or BUFFERED_PARTIAL
-//   mode is chosen, in multiple messages if the STREAMED mode is chosen, and not
-//   at all otherwise.
+// * Request body: Delivered if they are present and sent in a single message if
+//   the BUFFERED or BUFFERED_PARTIAL mode is chosen, in multiple messages if the
+//   STREAMED mode is chosen, and not at all otherwise.
 // * Request trailers: Delivered if they are present and if the trailer mode is set
 //   to SEND.
 // * Response headers: Contains the headers from the HTTP response. Keep in mind