wallet: migration bugfix, persist empty labels

addressbook records with no associated label could be
treated as change. And we don't want that for external
addresses.

Author: furszy

src/wallet/wallet.cpp

@@ -4059,10 +4059,10 @@ bool CWallet::ApplyMigrationData(MigrationData& data, bilingual_str& error)
         WalletBatch batch{wallet.GetDatabase()};
         for (const auto& [destination, addr_book_data] : wallet.m_address_book) {
             auto address{EncodeDestination(destination)};
-            auto label{addr_book_data.GetLabel()};
-            // don't bother writing default values (unknown purpose, empty label)
+            std::optional<std::string> label = addr_book_data.IsChange() ? std::nullopt : std::make_optional(addr_book_data.GetLabel());
+            // don't bother writing default values (unknown purpose)
             if (addr_book_data.purpose) batch.WritePurpose(address, PurposeToString(*addr_book_data.purpose));
-            if (!label.empty()) batch.WriteName(address, label);
+            if (label) batch.WriteName(address, *label);
         }
     };
     if (data.watchonly_wallet) persist_address_book(*data.watchonly_wallet);