Consider pallet owned account_id

[ilionic]

by h4x3rotab:

let (collection_id, nft_id) = Self::nft_mint(
    sender.clone().unwrap_or_default(),
    owner,
    collection_id,
    recipient,
    royalty,
    metadata,
)?;

When sender == None? If it means the origin is some admin (ProtocolOrigin), I suggest to use pallet's account id instead of the default account id (0x0), which is going to be deprecated soon due to some security concern.

Example of a pallet owned account_id:
https://github.com/paritytech/substrate/blob/d18705ee7b5b654d3947558fea6dbd0701ac3ecc/frame/society/src/lib.rs#L1733-L1735

[ilionic]

@h4x3rotab would you agree with simplified ensure all signed calls approach in #40 PR?

[h4x3rotab]

Looks perfect!