[Cashtab] Do not render empty parentheses for tickerless tokens in tx history

Summary: If a token has no ticker, do not render "()"

Test Plan:
npm test

I don't think it's worth adding a new mock to test this.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17002