[Cashtab] Better conditions for disabling amount in SendXec

Summary: txInfoFromUrl is either false or an object. So, we only wan to disable amount if it is both NOT false and has a value key not === 'null'

Test Plan:
npm start
Navigate to SendXec screen
amount input is not disabled

Navigate to http://localhost:3000/#/send?address=ecash:qzgyg3llme32kpru03ff7dq7lz8087ma6glfxngvp2&value=1012.71
amount input is disabled

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15189