When a DataFile is built through the derived builder (DataFileBuilder) and no split_offsets are given, this is still serialised as [] instead of null.

E.g. converting the Avro file to JSON it looks something like this:

   ...
    "split_offsets": {
      "array": []
    },

I suspect this is because the value within the DataFile is a Vec<i64> instead of an Option<Vec<i64>>, so we get an empty Vec instead of nothing.

I think this Option is the correct approach because the spec also states that split_offsets are optional and it looks like they're optional in the Java implementation too.