[Cashtab] Make sure send button is not disabled for airdrop txs

Summary: If a user generates an airdrop tx and then clicks 'copy to send screen', it will load with the send button disabled because formData.value is initialized as a string.

Test Plan:
npm test

Calculate an airdrop tx then click 'copy to send screen', note send button is not disabled

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14975