showcase broken case without explicit type in oneof pattern matching

[benedikt-bartscher]

Summary

this kind of match statement worked fine with python-betterproto, it seems to be a regression/breaking change in python-betterproto2

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
  • This change has an associated test.
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[benedikt-bartscher]

python-betterproto seems to raise AttributeError if you access oneof's which are not set which causes pythons pattern matching to check for the next case. This behavior has changed in python-betterproto2, where accessing unset oneof fields returns None. I guess this behavior is intended? We should probably add this change to a "breaking changes since betterproto1" list?