Consistent field nullability in the constructor and fromObject method

[Santalov]

The fromObject and constructor arguments became consistent in terms of nullability. To determine, if a field is nullable, the following function is used:

export function canBeCreatedWithoutValue(
  rootDescriptor: descriptor.FileDescriptorProto,
  fieldDescriptor: descriptor.FieldDescriptorProto,
) {
  return (
    isOptional(rootDescriptor, fieldDescriptor) ||
    // Both proto2 and proto3 don't track presence for maps and repeated fields.
    // https://github.com/protocolbuffers/protobuf/blob/main/docs/field_presence.md#presence-in-proto2-apis
    isMap(fieldDescriptor) ||
    isRepeated(fieldDescriptor)
  );
}

This also makes repeated and map fields nullable in constructor (they weren't).

[thesayyn]

Closing as there's been no activity on this.