2885c131b6b8ec1140e02ed8f2933c2ffbf41fd1 Qt: use [default wallet] as name for wallet with no name (Jonas Schnelli)

Pull request description:

Loading a wallet from a state where only the default wallet was active results in using an empty string for the initial/default wallet name.

This is a GUI only quick-fix that overrides wallet(s) with name "" to "[default wallet]". Does not affect `getwalletinfo` or `listwallets`.

Also, unsure if it should be fixed at a deeper level and if – instead of [default wallet] – it should use `wallet.dat` (the filename of the default wallet).

Tree-SHA512: 1d50dbb200b23df5ac53ce15aeb6453af4da354d6e6e53fe33ff075b477493254d6028b6d3569a7804b1aa616cb9a988a53de818937e37cdcb19cb70a90e2a88

Backport of Core PR13275
https://github.com/bitcoin/bitcoin/pull/13275/

Depends on D4236

make check
bitcoin-qt -> Help -> Debug -> Console
createwallet "testwallet"

In the upper left corner under the menu bar, the drop down menu should say [default wallet].
Close the console window.
The main window should still be dispaying the information for the default wallet.