[Cashtab] Fix NaN render bug in paybutton links without specified amount

Summary:
T3278

Prevent 'NaN' from being rendered for extension open by link with amount unspecified

Test Plan:
npm test

npm run extension
load in browser
test donation button at bottom of https://coin.dance/ and opens without NaN msg

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D14473