wallet, migration: Fix crash on empty wallet

Same as with a blank wallet, wallets with no legacy
records (i.e. empty, non-blank, watch-only wallet)
do not require to be migrated.

Author: pablomartin4btc

src/wallet/wallet.cpp

@@ -4534,13 +4534,13 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
         // First change to using SQLite
         if (!local_wallet->MigrateToSQLite(error)) return util::Error{error};
 
-        // Do the migration of keys and scripts for non-blank wallets, and cleanup if it fails
-        success = local_wallet->IsWalletFlagSet(WALLET_FLAG_BLANK_WALLET);
-        if (!success) {
+        // Do the migration of keys and scripts for non-empty wallets, and cleanup if it fails
+        if (HasLegacyRecords(*local_wallet)) {
             success = DoMigration(*local_wallet, context, error, res);
         } else {
             // Make sure that descriptors flag is actually set
             local_wallet->SetWalletFlag(WALLET_FLAG_DESCRIPTORS);
+            success = true;
         }
     }
 

test/functional/wallet_migration.py

@@ -445,6 +445,15 @@ def test_no_privkeys(self):
         # After migrating, the "keypool" is empty
         assert_raises_rpc_error(-4, "Error: This wallet has no available keys", watchonly1.getnewaddress)
 
+        self.log.info("Test migration of a watch-only empty wallet")
+        for idx, is_blank in enumerate([True, False], start=1):
+            wallet_name = f"watchonly_empty{idx}"
+            self.create_legacy_wallet(wallet_name, disable_private_keys=True, blank=is_blank)
+            _, watchonly_empty = self.migrate_and_get_rpc(wallet_name)
+            info = watchonly_empty.getwalletinfo()
+            assert_equal(info["private_keys_enabled"], False)
+            assert_equal(info["blank"], is_blank)
+
     def test_pk_coinbases(self):
         self.log.info("Test migration of a wallet using old pk() coinbases")
         wallet = self.create_legacy_wallet("pkcb")