Add a flag to allow empty structs in the thrift schema compat checker

This is useful as we migrate schemas, we want to be able to turn this check off.

**Reporter**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain
**Assignee**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain
#### PRs and other links:
- [PR-266](https://github.com/apache/parquet-mr/pull/266/)

_{**Note**: *This issue was originally created as [PARQUET-366](https://issues.apache.org/jira/browse/PARQUET-366). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}