qt: Refactor open date range to use std::optional

Summary:
This is a backport of core-gui#354
Depends on D18182

Test Plan: run bitcoin-qt, go to the transactions tab, at the top of the Date column select the various date filters

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18183