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/wallet.h

@@ -519,7 +519,7 @@ public:
 
     void Init()
     {
-        hashBlock = 0;
+        hashBlock = uint256();
         nIndex = -1;
         fMerkleVerified = false;
     }