rpc/wallet: initialize nFeeRequired explicitely to 0

Summary:
I do not think that we have the same issue as Core, but it is
still nice to see an explicit initialization to 0 in the code.

This is a backport of Core PR18274

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8805