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);
 

src/qt/test/wallettests.cpp

@@ -143,7 +143,7 @@ void TestGUI(interfaces::Node& node)
     bool firstRun;
     wallet->LoadWallet(firstRun);
     {
-        auto spk_man = wallet->GetLegacyScriptPubKeyMan();
+        auto spk_man = wallet->GetOrCreateLegacyScriptPubKeyMan();
         auto locked_chain = wallet->chain().lock();
         LOCK2(wallet->cs_wallet, spk_man->cs_KeyStore);
         wallet->SetAddressBook(GetDestinationForKey(test.coinbaseKey.GetPubKey(), wallet->m_default_address_type), "", "receive");