wallet: List sqlite wallets with empty string name

Although it is not explicitly possible to create a default wallet with
descriptors, it is possible to migrate a default wallet and have it end
up being a default wallet with descriptors. These wallets should be
listed by ListDatabases so that it appears in wallet directory listings
to avoid user confusion.

test/functional/wallet_migration.py

@@ -539,6 +539,7 @@ class WalletMigrationTest(BitcoinTestFramework):
         assert_equal(info["format"], "sqlite")
 
         walletdir_list = wallet.listwalletdir()
+        assert {"name": info["walletname"]} in walletdir_list["wallets"]
 
         # Check backup existence and its non-empty wallet filename
         backup_filename = f"default_wallet_{curr_time}.legacy.bak"