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/rpcblockchain.cpp

@@ -70,7 +70,7 @@ Object blockToJSON(const CBlock& block, const CBlockIndex* blockindex, bool txDe
         if(txDetails)
         {
             Object objTx;
-            TxToJSON(tx, uint256(0), objTx);
+            TxToJSON(tx, uint256(), objTx);
             txs.push_back(objTx);
         }
         else