[Cashtab] Do not disable amount field if null is passed

Summary:
Amount field should not be disabled if intent was to allow user to customize amount

Context: The way txInfoFromParams is being handled is currently in the middle of a significant refactor to handle bip21 spec updates.

This patch solves an immediate issue.

Test Plan:
npm run extension
Test at https://proofofwriting.com/204/

Also make sure it still works at ecash.land

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15185