Remove error when no fields are set in oneof

[AdrienVannson]

Summary

Fix #604

Methods such as parse are often called on a new object: value = cls().parse(value). However, is the message contains oneof fields, this used to make the validator _validate_field_groups fail when using pydantic.

I removed the check that at least one value of the oneof is defined. This is coherent with gRPC's documentation: "If you have a message with many fields and where at most one field will be set at the same time, you can enforce this behavior and save memory by using the oneof feature." and "You can check which value in a oneof is set (if any) using a special case() or WhichOneof() method"

Checklist

• If code changes were made then they have been tested.
• This PR fixes an issue.

[AdrienVannson]

I forget to mention: this problem makes some of the tests from test_inputs.py fail. The problem only appears when plugin_output_package is manually set to "tests.output_betterproto_pydantic", which is not the case by default. Once this problem (and at least another one) is fixed, I will send a PR to also execute the input tests with Pydantic.

[Gobot1234]

I forget to mention: this problem makes some of the tests from test_inputs.py fail. The problem only appears when plugin_output_package is manually set to "tests.output_betterproto_pydantic", which is not the case by default. Once this problem (and at least another one) is fixed, I will send a PR to also execute the input tests with Pydantic.

That would be a huge help actually