wallet: default wallet migration, modify inconvenient backup filename

On default legacy wallets, the backup filename starts with an "-" due
to the wallet name being empty. This is inconvenient for systems who
treat what follows the initial "-" character as flags.

src/wallet/wallet.cpp

@@ -4336,7 +4336,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
 
     // Make a backup of the DB
     fs::path this_wallet_dir = fs::absolute(fs::PathFromString(local_wallet->GetDatabase().Filename())).parent_path();
-    fs::path backup_filename = fs::PathFromString(strprintf("%s-%d.legacy.bak", wallet_name, GetTime()));
+    fs::path backup_filename = fs::PathFromString(strprintf("%s_%d.legacy.bak", (wallet_name.empty() ? "default_wallet" : wallet_name), GetTime()));
     fs::path backup_path = this_wallet_dir / backup_filename;
     if (!local_wallet->BackupWallet(fs::PathToString(backup_path))) {
         if (was_loaded) {

test/functional/wallet_migration.py

@@ -529,11 +529,20 @@ class WalletMigrationTest(BitcoinTestFramework):
         self.log.info("Test migration of the wallet named as the empty string")
         wallet = self.create_legacy_wallet("")
 
-        self.migrate_wallet(wallet)
+        # Set time to verify backup existence later
+        curr_time = int(time.time())
+        wallet.setmocktime(curr_time)
+
+        res = self.migrate_wallet(wallet)
         info = wallet.getwalletinfo()
         assert_equal(info["descriptors"], True)
         assert_equal(info["format"], "sqlite")
 
+        # Check backup existence and its non-empty wallet filename
+        backup_path = self.nodes[0].wallets_path / f'default_wallet_{curr_time}.legacy.bak'
+        assert backup_path.exists()
+        assert_equal(str(backup_path), res['backup_path'])
+
     def test_direct_file(self):
         self.log.info("Test migration of a wallet that is not in a wallet directory")
         wallet = self.create_legacy_wallet("plainfile")