psbt: Check sighash types in SignPSBTInput and take sighash as optional

src/rpc/util.cpp

@@ -378,10 +378,10 @@ UniValue DescribeAddress(const CTxDestination& dest)
  *
  * @pre The sighash argument should be string or null.
 */
-int ParseSighashString(const UniValue& sighash)
+std::optional<int> ParseSighashString(const UniValue& sighash)
 {
     if (sighash.isNull()) {
-        return SIGHASH_DEFAULT;
+        return std::nullopt;
     }
     const auto result{SighashFromStr(sighash.get_str())};
     if (!result) {