JSON serialization of default value for fields with explicit presence

[antongrbin]

Motivation

Fixes https://github.com/streem/pbandk/issues/235

Changes

• Change the behavior for JSON serialization for fields with explicit presence.
• If unset -> don't emit on the wire.
• If set -> emit on the wire, even if the value is default (regardless of the jsonConfig.outputDefaultValues)

This is consistent with the following: https://github.com/protocolbuffers/protobuf/blob/main/docs/field_presence.md#semantic-differences

This PR also changes behavior of outputDefaultValues for unset nested message fields. Before we were outputting null and now we don't output any value. This is consistent with c++, python and java, as explained here: https://github.com/noom/protobuf/pull/5

Tested

Run all unit tests in runtime