Replace direct use of 0 with SetNull and IsNull

Replace x=0 with .SetNull(),
x==0 with IsNull(), x!=0 with !IsNull().
Replace uses of uint256(0) with uint256().

src/walletdb.cpp

@@ -439,7 +439,7 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
             }
             CKey key;
             CPrivKey pkey;
-            uint256 hash = 0;
+            uint256 hash;
 
             if (strType == "key")
             {
@@ -464,7 +464,7 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
 
             bool fSkipCheck = false;
 
-            if (hash != 0)
+            if (!hash.IsNull())
             {
                 // hash pubkey/privkey to accelerate wallet load
                 std::vector<unsigned char> vchKey;