fix(channeld): handle NULL short_channel_id in AWAITING_LOCKIN state

[kwsantiago]

Fixes #7222 crash when reconnecting channels in CHANNELD_AWAITING_LOCKIN state without short_channel_id.

The issue occurred when channels stuck in AWAITING_LOCKIN state (with missing funding tx) attempted to reconnect. The code passed a zero-initialized scid struct to towire_channeld_init, causing a crash when accessing short_channel_id.u64.

Changes:

• Modified peer_start_channeld to use pointer for scid instead of struct
• Updated channeld wire protocol to handle optional short_channel_id
• Added defensive check in connect_activate_subd for NULL scid
• channeld now falls back to local_alias when has_funding_short_id is false