wallet: Check for descriptors flag before migration

Previously we would check that there is no LegacySPKM in order to
determine whether a wallet is already a descriptor wallet and doesn't
need to be migrated. However blank legacy wallets will also not have a
LegacySPKM, so we need to be checking for the descriptors flag instead.

Author: achow101

src/wallet/wallet.cpp

@@ -4213,7 +4213,7 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(const std::string& walle
     }
 
     // Before anything else, check if there is something to migrate.
-    if (!local_wallet->GetLegacyScriptPubKeyMan()) {
+    if (local_wallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS)) {
         return util::Error{_("Error: This wallet is already a descriptor wallet")};
     }