Helper for pagination conventions of streaming RPCs

The general convention for streamed responses with the pagination protocol is that the PaginationCursor attribute must be set on the first response and only the first response. As seen in #91 (closed), an empty PaginationCursor is not what is expected there.

This is hard to get right because of this subtle difference between the Python and Ruby gRPC libraries.

Python gRPC

A missing message field is returned back as the empty message:

>>> resp = GetTreeEntriesResponse()
>>> resp.pagination_cursor is None
False
>>> resp.pagination_cursor == PaginationCursor()
True

Despite this, fortunately, there is still a difference in Python between not setting and setting an empty value

>>> resp.HasField('pagination_cursor')
False
>>> resp2 = GetTreeEntriesResponse(pagination_cursor=PaginationCursor())
>>> resp2.HasField('pagination_cursor')
True

Also not setting and setting None are equivalent (can be handy for code uniformity)

>>> resp3 = GetTreeEntriesResponse(pagination_cursor=None)
>>> resp3.HasField('pagination_cursor')
False

Ruby gRPC

A missing message is returned back as nil

[2] pry(main)> resp = Gitaly::GetTreeEntriesResponse.new()
=> <Gitaly::GetTreeEntriesResponse: entries: []>
[3] pry(main)> resp.pagination_cursor
=> nil