[Cashtab] support valid addresses if no prefix included

Summary: Adding prefix validation on Friday broke case of valid cash address with no prefix. This accepts valid addresses without a prefix, as provided by Coinex.

Test Plan: npm test includes this test case. Can try depositing at your Coinex BCHA address to confirm with real money.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8944