Refactor: Allow LegacyScriptPubKeyMan to be null

In CWallet::LoadWallet, use this to detect and empty wallet with no keys

This commit does not change behavior.

src/qt/test/addressbooktests.cpp

@@ -59,6 +59,7 @@ void TestAddAddressesToSendBook(interfaces::Node& node)
 {
     TestChain100Setup test;
     std::shared_ptr<CWallet> wallet = std::make_shared<CWallet>(node.context()->chain.get(), WalletLocation(), WalletDatabase::CreateMock());
+    wallet->SetupLegacyScriptPubKeyMan();
     bool firstRun;
     wallet->LoadWallet(firstRun);