[ecash-wallet] Patch ts issue to stay consistent with defined interface

Summary:
If an app uses ecash-wallet as a dep and has strict type checking, it will throw an error on this.

Properly speaking we only want to have a missingTokens key if there are missing tokens; we do not really want missingTokens: undefined

Patch.

Test Plan: npm test and CI tests

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18839