[backport#15957] Show loaded wallets as disabled in open menu instead of nothing

Summary:
https://github.com/bitcoin/bitcoin/pull/15957/commits/c3ef63a52f304a600fff1f9c7caa5cb804d41d43

This is a backport of Core PR15957

Test Plan:

ninja check
./src/qt/bitcoin-qt -regtest

Focus the Open wallet menu item, see that [default wallet] appears, greyed out because it is loaded.

Empty out the std::vector<std::string> wallets right before the for-loop that iterates through it, compile and run the above again, see that focusing the Open wallet menu item shows No wallets available

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6163