wallet: Fix typo in assert that is compile-time true

Summary:

Commit 92bcd70 (D8667] presumably added a check that a dest of type CNoDestination implies an empty scriptChange.

However, it accidentally checked for boost::variant::empty, which always returns false:

This is a backport of Core PR18853

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9081