[Cashtab] Do not disable amount if value from params is undefined

Summary:
Cashtab can parse a tx from URL params address and value. We will deprecate this so that Cashtab parses txs from bip21 strings in the URL.

For now, some PayButton txs pass value=undefined. In this case, the amount input will be disabled.

Do not disable the amount input for this case.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15239