Improve test coverage in `datafusion-proto`

[andygrove]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
The datafusion-proto crate provides serde of logical plans and expressions. We have limited test coverage and it would be good to have more comprehensive tests.

Describe the solution you'd like
Write tests to cover all operators and expressions. We could also consider using fuzz testing as described in https://howqueryengineswork.com/14-testing.html

Describe alternatives you've considered
None

Additional context
None

[Jefffrey]

We could look into doing something like #16183 but for proto roundtrip