Add Avro file for null test case

[nenorbot]

To be used for the changes in apache/datafusion#6307

file contents:

❯ avro-tools tojson data/avro/alltypes_nulls_plain.avro
{"string_col":null,"int_col":null,"bool_col":null,"bigint_col":null,"float_col":null,"double_col":null,"bytes_col":null}

[alamb]

Thanks @nenorbot -- looks good to me and I think a 603 byte file is perfect

I wonder if you could update the https://github.com/apache/arrow-testing/tree/master/data/avro#readme readme file as well with the information about this file (perhaps the PR description)

[nenorbot]

I wonder if you could update the https://github.com/apache/arrow-testing/tree/master/data/avro#readme readme file as well with the information about this file (perhaps the PR description)

@alamb Done, added a note about the new file

[alamb]

Thank you!