[Cashtab] Patch proptypes in BalanceHeader

Summary:
lingering copypasta prop type and proptypes does not support | syntax for 'or'

If they are not required props, just use PropTypes.number, which supports null or undefined.

Test Plan:
npm start and nothing in the dev console about invalid prop types

Note: proptypes are a dev tool, this doesn't impact the built app

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15260