Proto-Parquet cannot write a schema with an empty group

This is valid protobuf:

 
```java

message Something {
    EmptyMessage outerEmptyMessage = 1;
}

message EmptyMessage {
}
```
However when we try to convert this to Parquet we get:
```java

org.apache.parquet.schema.InvalidSchemaException: Cannot write a schema with an empty group: optional group outerEmptyMessage = 1 {
}
```
Is this a limitation of the Parquet format, or just a bug?

If the former, can we break earlier when generating the schema out of Protobuf, and not when trying to write the content to the already generated schema?

**Reporter**: [André Pinto](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=andresp)

_{**Note**: *This issue was originally created as [PARQUET-1240](https://issues.apache.org/jira/browse/PARQUET-1240). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by reproducing the reported protobuf example with an empty nested message and follow the protobuf-to-Parquet schema-generation path. Determine whether the generated empty group is unsupported by Parquet or produced incorrectly, and make the failure occur at schema generation if earlier validation is required.

Written by the indexing model from the issue text.