[Cashtab] Patch proptypes issues

Summary:
Patch proptypes errors

• SendToken initially passes '' to decimals, so accept this in proptypes
• the max prop is a number for the cashtab msg field, and it works fine, so update proptypes to accept this
• undefined is not a proptype, if a proptype is not isRequired, then the component accepts undefined implicitly

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15866