always return endcursor - fixes #288

Went with empty string so a dev can always expect to deal with one type of data. In either case, a user could do `if (endCursor) { runQueryAgain() }`, but if they for some reason intended to do string-only things, or were just blindly passing the value to the query builder, we'd have to make sure the `query.proto.start` method handles null vs '' vs 'actualvalue'