Add Avro file for null test case (#90)

To be used for the changes in
https://github.com/apache/arrow-datafusion/pull/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}
```
tree: aa2b36fb65aa8b73512223847c905d27879b0852
  1. data/
  2. LICENSE.txt
  3. README.md
README.md