[backport#15153] Interfaces: Avoid interface instance if wallet is null

Summary:
bitcoin/bitcoin@4c8982a

---

Depends on D6119

This is a partial backport of Core [[bitcoin/bitcoin#15153 | PR15153]]

Test Plan:
  ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6121

src/interfaces/wallet.cpp

@@ -521,7 +521,7 @@ namespace {
 } // namespace
 
 std::unique_ptr<Wallet> MakeWallet(const std::shared_ptr<CWallet> &wallet) {
-    return std::make_unique<WalletImpl>(wallet);
+    return wallet ? std::make_unique<WalletImpl>(wallet) : nullptr;
 }
 
 std::unique_ptr<ChainClient>