wallet: Fix for exported confirmation field in payment to self transactions

Summary:
When exporting transactions as a CSV file, the "confirmed" column always showed True for transactions sent to self, even when not confirmed yet.
This fixes this bug.

This is a backport of core#14988

Test Plan:
ninja && src/qt/bitcoin-qt

Send a transaction to yourself, export the transactions list as CSV before the new transaction is included in a block, and verify that it has "false" in the "Confirmed" column.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9929