Only trigger conflicting oneof if a value is decoded.
Only trigger conflicting oneof if a value is decoded.

This lets things like JSON that can have `null` as the value not count as
setting the field and thus not causing the conflict per the new conformance
test.

Fixes #1133

cburrows/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf