wallet: allow mintxfee=0

[delta1]

Allow -mintxfee=0, so the elements wallet can create transactions with 0 value fee outputs.

Elements can already accept 0 value fees with -minrelaytxfee=0 and -blockmintxfee=0, but before this change the wallet would not create a transaction with 0 feerate.

Note: this check for 0 was inherited from upstream and subsequently removed in https://github.com/bitcoin/bitcoin/commit/f11eb1fe279c4a92e1bfc2c139e8838c73459d12