Fix issue with matchFields refactor

Author: julian-elastic

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/plan/physical/HashJoinExec.java

@@ -65,7 +65,7 @@ private HashJoinExec(StreamInput in) throws IOException {
     public void writeTo(StreamOutput out) throws IOException {
         super.writeTo(out);
         // TODO: clean up, we used to read the match fields here.
-        out.writeNamedWriteableCollection(null);
+        out.writeNamedWriteableCollection(leftFields);
         out.writeNamedWriteableCollection(leftFields);
         out.writeNamedWriteableCollection(rightFields);
         out.writeNamedWriteableCollection(addedFields);