[ecashaddrjs] Fix types for isValidCashAddress

Summary: TypeScript gives an error when optionalPrefix in not defined when calling isValidCashAddress, this PR allows both undefined or explicit false for no prefixes, or a user passed string as prefix

Test Plan: npm test

Reviewers: bytesofman, Fabien, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: bytesofman, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16331