[Backport]gui: hide HD & encryption icons when no wallet loaded

Summary:
The HD and encrypt status bar icons were not hidden after closing all wallets in bitcoin-qt.

This is a Backport of Bitcoin Core PR17935 and PR18007
The second PR is a fix for a bug introduced by the first.

Test Plan:
Reproduce the issue before applying the commits: close all wallets in bitcoin-qt and check that status icons remain visible.
Build, install, create 2 encrypted wallets, check that the icons now disappear when all wallets are closed, but remains visible after closing a wallet if a second wallet is still open.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7540