Fix start with the -min option

Summary:
PR description:

When GUI starts with the -min option, the `Minimize to tray instead of
the taskbar` option works as expected now.

Backport of Core PR14517

Test Plan:

ninja && ninja check
src/qt/bitcoin-qt -min

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7841