wallet: fix, detect blank legacy wallets in IsLegacy

Blank legacy wallets do not have active SPKM. They can
only be detected by checking the descriptors' flag or
the db format.

This enables the migration of blank legacy wallets in
the GUI.

Author: furszy

src/wallet/wallet.cpp

@@ -1037,22 +1037,22 @@ bool CWallet::IsSpentKey(const CScript& scriptPubKey) const
     if (IsAddressPreviouslySpent(dest)) {
         return true;
     }
-    if (IsLegacy()) {
-        LegacyScriptPubKeyMan* spk_man = GetLegacyScriptPubKeyMan();
-        assert(spk_man != nullptr);
-        for (const auto& keyid : GetAffectedKeys(scriptPubKey, *spk_man)) {
-            WitnessV0KeyHash wpkh_dest(keyid);
-            if (IsAddressPreviouslySpent(wpkh_dest)) {
-                return true;
-            }
-            ScriptHash sh_wpkh_dest(GetScriptForDestination(wpkh_dest));
-            if (IsAddressPreviouslySpent(sh_wpkh_dest)) {
-                return true;
-            }
-            PKHash pkh_dest(keyid);
-            if (IsAddressPreviouslySpent(pkh_dest)) {
-                return true;
-            }
+
+    LegacyScriptPubKeyMan* spk_man = GetLegacyScriptPubKeyMan();
+    if (!spk_man) return false;
+
+    for (const auto& keyid : GetAffectedKeys(scriptPubKey, *spk_man)) {
+        WitnessV0KeyHash wpkh_dest(keyid);
+        if (IsAddressPreviouslySpent(wpkh_dest)) {
+            return true;
+        }
+        ScriptHash sh_wpkh_dest(GetScriptForDestination(wpkh_dest));
+        if (IsAddressPreviouslySpent(sh_wpkh_dest)) {
+            return true;
+        }
+        PKHash pkh_dest(keyid);
+        if (IsAddressPreviouslySpent(pkh_dest)) {
+            return true;
         }
     }
     return false;
@@ -1625,7 +1625,9 @@ isminetype CWallet::IsMine(const CScript& script) const
     }
 
     // Legacy wallet
-    if (IsLegacy()) return GetLegacyScriptPubKeyMan()->IsMine(script);
+    if (LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan()) {
+        return spkm->IsMine(script);
+    }
 
     return ISMINE_NO;
 }
@@ -3558,7 +3560,8 @@ std::set<ScriptPubKeyMan*> CWallet::GetScriptPubKeyMans(const CScript& script) c
     Assume(std::all_of(spk_mans.begin(), spk_mans.end(), [&script, &sigdata](ScriptPubKeyMan* spkm) { return spkm->CanProvide(script, sigdata); }));
 
     // Legacy wallet
-    if (IsLegacy() && GetLegacyScriptPubKeyMan()->CanProvide(script, sigdata)) spk_mans.insert(GetLegacyScriptPubKeyMan());
+    LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan();
+    if (spkm && spkm->CanProvide(script, sigdata)) spk_mans.insert(spkm);
 
     return spk_mans;
 }
@@ -3588,7 +3591,8 @@ std::unique_ptr<SigningProvider> CWallet::GetSolvingProvider(const CScript& scri
     }
 
     // Legacy wallet
-    if (IsLegacy() && GetLegacyScriptPubKeyMan()->CanProvide(script, sigdata)) return GetLegacyScriptPubKeyMan()->GetSolvingProvider(script);
+    LegacyScriptPubKeyMan* spkm = GetLegacyScriptPubKeyMan();
+    if (spkm && spkm->CanProvide(script, sigdata)) return spkm->GetSolvingProvider(script);
 
     return nullptr;
 }
@@ -3845,11 +3849,7 @@ void CWallet::DeactivateScriptPubKeyMan(uint256 id, OutputType type, bool intern
 
 bool CWallet::IsLegacy() const
 {
-    if (m_internal_spk_managers.count(OutputType::LEGACY) == 0) {
-        return false;
-    }
-    auto spk_man = dynamic_cast<LegacyScriptPubKeyMan*>(m_internal_spk_managers.at(OutputType::LEGACY));
-    return spk_man != nullptr;
+    return !IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS);
 }
 
 DescriptorScriptPubKeyMan* CWallet::GetDescriptorScriptPubKeyMan(const WalletDescriptor& desc) const