[Task]: Add property-based tests for Beam Schema and encoding that test nullability and structured types

### What needs to happen?

PR https://github.com/apache/beam/pull/22233 adds tests for generated schema encoding and so on, but does not test structured types and doesn't test nullability either.

A great follow up would be to test these.

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Another follow up would be to add tests that are parameterized by a protobuf representing a Beam schema (the tests in https://github.com/apache/beam/pull/22233 are parameterized by an autogenerated Python type).

### Issue Priority

Priority: 2

### Issue Component

Component: cross-language