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/net.cpp

@@ -1949,7 +1949,7 @@ CNode::CNode(SOCKET hSocketIn, CAddress addrIn, std::string addrNameIn, bool fIn
     nRefCount = 0;
     nSendSize = 0;
     nSendOffset = 0;
-    hashContinue = 0;
+    hashContinue = uint256();
     nStartingHeight = -1;
     fGetAddr = false;
     fRelayTxes = false;