New board grammar doesn't allow creation of empty board

[Mearman]

in updating my tests for https://github.com/breadboard-ai/breadboard/pull/587/#discussion_r1476743274 I discovered this, which is preventing me from creating an empty board.

 const emptyBoard = await board().serialize();

https://github.com/breadboard-ai/breadboard/blob/b5577943bdd0956bed3874244b34ea80f1589eaa/packages/breadboard/src/new/grammar/board.ts#L87-L88