[Cashtab] BalanceHeaderFiat component handles null value for fiatPrice

Summary: Related to task T2314. Removed conditional logic from Send.js which handled rendering the BalanceHeaderFiat component and moved it into the BalanceFiat component.

Test Plan:
cd web/cashtab
npm start
from the wallet tab, observe the BalanceHeaderFiat component.
ctrl + refresh the page, observe the BalanceHeaderFiat component render as the api fetches the fiatPrice.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11199