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/coins.h

@@ -297,7 +297,7 @@ struct CCoinsStats
     uint256 hashSerialized;
     CAmount nTotalAmount;
 
-    CCoinsStats() : nHeight(0), hashBlock(0), nTransactions(0), nTransactionOutputs(0), nSerializedSize(0), hashSerialized(0), nTotalAmount(0) {}
+    CCoinsStats() : nHeight(0), nTransactions(0), nTransactionOutputs(0), nSerializedSize(0), nTotalAmount(0) {}
 };