[Cashtab] Patch airdrop blank screen bug

Summary: Patching issue from T3626 when routing from the Airdrop component to Send. This is because in this specific route the cashtabMsg form data is not initialized leading to an exception being thrown by getCashtabMsgByteCount.

Test Plan:
npm start
Go to Token Tab and chose token
Go to Airdrop Calculator
Calculate Airdrop
Copy to send Screen
Click on "Cashtab Message" toggle and ensure no blank screen

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16711