[ecash-wallet] Do not add unnecessary postage

Summary: It's possible a tx built with the NO_SATS strategy and intended for postage does not actually need it. We should check for this before we start to add fuel utxos.

Test Plan: npm test, CI

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18920