fix: don't set zero size if size is unknown (#1141)

Sending zero instead of null confuses other sdks (at least rust), e.g.
[here](https://github.com/livekit/rust-sdks/blob/e37adc1dc4cb90322733fcb0fe1155262f309ff1/livekit-data-stream/src/incoming.rs#L325)

---------

Co-authored-by: Hiroshi Horie <548776+hiroshihorie@users.noreply.github.com>

Author: holzgeist

.changes/fix-data-stream-size

@@ -0,0 +1 @@
+patch type="fixed" "Omit data stream totalLength when size is unknown"

lib/src/participant/local.dart

@@ -1062,7 +1062,7 @@ extension DataStreamParticipantMethods on LocalParticipant {
       mimeType: info.mimeType,
       topic: info.topic,
       timestamp: Int64(timestamp),
-      totalLength: Int64(options?.totalSize ?? 0),
+      totalLength: options?.totalSize != null ? Int64(options!.totalSize!) : null,
       attributes: options?.attributes.entries,
       textHeader: lk_models.DataStream_TextHeader(
         version: options?.version,
@@ -1150,7 +1150,7 @@ extension DataStreamParticipantMethods on LocalParticipant {
     );
 
     final header = lk_models.DataStream_Header(
-      totalLength: Int64(info.size),
+      totalLength: options?.totalSize != null ? Int64(options!.totalSize!) : null,
       mimeType: info.mimeType,
       streamId: streamId,
       topic: options?.topic,