[Cashtab] Improve error handling in validation.js

Summary: Added error handling for null and empty string inputs for the isValidEtokenAddress and isValidXecAddress functions within validation.js

Test Plan:
npm test
ensure validation.test.js test suites pass successfully

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10820