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/bitcoin-tx.cpp

@@ -315,7 +315,7 @@ static bool findSighashFlags(int& flags, const string& flagStr)
 uint256 ParseHashUO(map<string,UniValue>& o, string strKey)
 {
     if (!o.count(strKey))
-        return 0;
+        return uint256();
     return ParseHashUV(o[strKey], strKey);
 }
 
@@ -485,7 +485,7 @@ static void MutateTx(CMutableTransaction& tx, const string& command,
 static void OutputTxJSON(const CTransaction& tx)
 {
     UniValue entry(UniValue::VOBJ);
-    TxToUniv(tx, 0, entry);
+    TxToUniv(tx, uint256(), entry);
 
     string jsonOutput = entry.write(4);
     fprintf(stdout, "%s\n", jsonOutput.c_str());