ts-proto: Repeated fields can be undefined

It appears that a repeated field like this one:

   repeated Document documents = 2;

Simply gives this as output:

   documents: Document[];

But, if you use the standard NodeJS protobufjs serialization without adding defaults: true a deserialized message won’t actually contain a documents field.

So it seems more appropriate to output:

   documents: Document[] | undefined;

Or even:

   documents?: Document[];

Of course, it’s always possible to add an option to turn this on or off using a setting similar to --ts_proto_opt=stringEnums=true (which BTW does nicely fit the the enums: String override on the protobufjs (de)serializer).

I could help on a PR for this, but first wanted to check what the preferred solution could be.