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.*}