Set ReplicationRequest.internalShardId to null

This commit sets ReplicationRequest.internalShardId to null when the
stream indicates that no ShardId is present in the stream.

Additionally, the use of StreamOutput#writeOptionalStreamable is
changed to be explicit for clarity since the use of
StreamInput#readOptionalStreamable is not possible due to the
no-argument constructor on ShardId being private.

core/src/main/java/org/elasticsearch/action/support/replication/ReplicationRequest.java

@@ -155,6 +155,8 @@ public class ReplicationRequest<T extends ReplicationRequest> extends ActionRequ
         super.readFrom(in);
         if (in.readBoolean()) {
             internalShardId = ShardId.readShardId(in);
+        } else {
+            internalShardId = null;
         }
         consistencyLevel = WriteConsistencyLevel.fromId(in.readByte());
         timeout = TimeValue.readTimeValue(in);
@@ -164,7 +166,12 @@ public class ReplicationRequest<T extends ReplicationRequest> extends ActionRequ
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
-        out.writeOptionalStreamable(internalShardId);
+        if (internalShardId != null) {
+            out.writeBoolean(true);
+            internalShardId.writeTo(out);
+        } else {
+            out.writeBoolean(false);
+        }
         out.writeByte(consistencyLevel.id());
         timeout.writeTo(out);
         out.writeString(index);