wallet: Avoid null pointer deref when cleaning up migratewallet

If migratewallet fails, we do a cleanup which removes the watchonly and
solvables wallets if they were created. However, if they were not, their
pointers are nullptr and we don't check for that, which causes a
segfault during the cleanup. So check that they aren't nullptr before
cleaning them up.

src/wallet/wallet.cpp

@@ -4093,8 +4093,8 @@ util::Result<MigrationResult> MigrateLegacyToDescriptor(std::shared_ptr<CWallet>
 
         // Make list of wallets to cleanup
         std::vector<std::shared_ptr<CWallet>> created_wallets;
-        created_wallets.push_back(std::move(res.watchonly_wallet));
+        if (res.watchonly_wallet) created_wallets.push_back(std::move(res.watchonly_wallet));
-        created_wallets.push_back(std::move(res.solvables_wallet));
+        if (res.solvables_wallet) created_wallets.push_back(std::move(res.solvables_wallet));
 
         // Get the directories to remove after unloading
         for (std::shared_ptr<CWallet>& w : created_wallets) {