Nil values in generated protobufs

[matejpavlovic]

The Pb() method of types generated from protobufs panics if there is a nil value in some of the object's fields.
The generated code should be adapted so that nil values are faithfully reproduced.

At least, even without support for nil values, at least make the conversions happen in one place and gracefully ignore those conversions that could fail due to some other node being faulty.