[Cashtab] If timeFirstSeen is 0, use block timestamp in tx history

Summary:
Use block timestamp for tx timestamp in history if data.timeFirstSeen is unavailable

Patches a bug where a tx with no timeFirstSeen that is unconfirmed would fail to render.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15215