Parquet protobuf: List wrapper should be optional

Following  , parquet-protobuf will conform to the specs[1] regarding the definition of lists and will adopt the 3-level structure:
```java

*required* group my_list (LIST) {
repeated group list {
optional binary element (UTF8);
}
}
```
The first level has the repetition level **required**, however this won't allow making the distinction between a null list and an empty one.

This ticket is for changing the root level repetition to **optional** as following:
```java

*optional* group my_list (LIST) {
repeated group list {
optional binary element (UTF8);
}
}
```
This will allow differentiating between null and empty lists.

[1] https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#lists

**Reporter**: [Benoit Hanotte](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=b.hanotte)

_{**Note**: *This issue was originally created as [PARQUET-1216](https://issues.apache.org/jira/browse/PARQUET-1216). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by locating the parquet-protobuf implementation of list schemas and compare it with the Parquet lists specification linked in the issue and the referenced parquet-mr change. Done means the root LIST group is optional, while the nested list and element structure remains as shown, allowing null and empty lists to be distinguished.

Written by the indexing model from the issue text.