Correct primitive_null.value (#86)

Author: alamb

variant/README.md

@@ -49,7 +49,7 @@ The files in this directory were initially generated by running the [`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`) 
+### Modification 1: Created metadata and value 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. 
@@ -62,5 +62,12 @@ The metadata for `primitive_null` should be the same 3 bytes as other primitive
 cp primitive_int8.metadata primitive_null.metadata
 ```
 
+The value for a primitive should be a `value_header` and no `value_data`,
+resulting in a single `0` byte:
+
+```shell
+echo -n 'a' | tr a '\0' > primitive_null.value
+```
+
 [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