Add primitive_null metadata for example Variant values (#84)

* Add primitive_null metadata

* Update variant/README.md

Co-authored-by: Fokko Driesprong <fokko@apache.org>

* Update variant/README.md

---------

Co-authored-by: Fokko Driesprong <fokko@apache.org>

Author: alamb

variant/README.md

@@ -45,8 +45,22 @@ Each example consists of 2 files:
 
 ## Regenerating these files
 
-The files were generated by running the [`regen.py`](regen.py) script that uses Apache Spark to 
-generate the files.
+The files in this directory were initially generated by running the [`regen.py`](regen.py) 
+script which used Apache Spark to generate the files. The files have been subsequently modified
+when necessary to ensure that they conform to the Parquet spec.
+
+### Modification 1: Created metadata for `primitive_null` as a single byte (`0x01`) 
+
+Per <https://github.com/apache/parquet-testing/issues/81>, Spark did not generate
+any metadata for `null` and left `primitive_null.metadata` empty. 
+The metadata for `primitive_null` should be the same 3 bytes as other primitive types 
+* header = `0x01`
+* dictionary_size = `0x00`
+* `dictionary_size + 1 = 1` byte values: `0x00` 
+
+```shell
+cp primitive_int8.metadata primitive_null.metadata
+```
 
 [Variant]: https://github.com/apache/parquet-format/blob/master/VariantEncoding.md
 [primitive types listed in the spec]: https://github.com/apache/parquet-format/blob/master/VariantEncoding.md#value-data-for-primitive-type-basic_type0