Nonce in out-of-order messages

[Hristo Georgiev]

Hello
I’m working on file transfer protocol inspired by Noise and Wireguard over UDP  with static keys so the handshake pattern is going to be Noise_IK_25519_ChaChaPoly_BLAKE2s  or KK. In general i’m trying to follow NoiseSocket spec but adapted to UDP and i was wondering where to place the ‘nonce’ into packet to extend the protocol as described into p.11.4 in NoiseProtocol spec http://www.noiseprotocol.org/noise.html#out-of-order-transport-messages .  So I had a look into WireGuard implementation (which I use a lot in multiple sites, mostly on EdgeRouters)
so what i found in noise.c line 445 message_decrypt:

chacha20poly1305_decrypt(dst_plaintext, src_ciphertext, src_len,
				      hash, NOISE_HASH_LEN,
				      0 /* Always zero for Noise_IK */, key))

Correct me if I’m mistaken, I’m not C programer nor cryptographer, I think this zero here is the Nonce. If that’s the case what security sacrifices and the chance for replay attacks are?
Would it be Ok if I also hardcode the Nonce to 0 in my implementation?

Thank you,
Hristo