[Cashtab] Exclude slp utxos if tokenQty is 0

Summary: If an slp utxo is returned with tokenQty === '0', it should not be included in the app's token utxo set. This diff adds this rule and a new unit test confirming it.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9362