make minter for instantiate msg optional

[taitruong]

Rather prefer having minter: Option<String>:

cw-nfts/contracts/cw721-base/src/msg.rs

Lines 8 to 18 in 177a993

	pub struct InstantiateMsg {
	/// Name of the NFT contract
	pub name: String,
	/// Symbol of the NFT contract
	pub symbol: String,
	/// The minter is the only one who can create new NFTs.
	/// This is designed for a base NFT that is controlled by an external program
	/// or contract. You will likely replace this with custom logic in custom NFTs
	pub minter: String,
	}

In case of none, info.sender should be minter. Makes it easier for other contracts (e.g. minter) re-using this msg.