Revisit `submit_message()` REST API logic

[samuel-rufi]

Description

With the latest stardust changes, the milestone nonce is 0 and needs to be handled separately https://github.com/iotaledger/bee/commit/f940e297cb8b138d916ad6505c66317c20ebb360

The submit_message() API may reject zero-nonce values; the submit_message() API logic should therefore be revisited. https://github.com/iotaledger/bee/blob/shimmer-develop/bee-api/bee-rest-api/src/endpoints/routes/api/v2/submit_message.rs#L156