[bip21] Allow unprefixed ecash addresses

Summary: Since there are practical limits in what can fit into a QR code, allow prefixless addresses.

Test Plan: Proofread, comment on any issues

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D15200