[rpc] walletcreatefundedpsbt: allow inputs to be null

This is of neglible use here, but it allows new RPC methods to take outputs as their first argument and make inputs optional.

src/rpc/rawtransaction_util.cpp

@@ -21,10 +21,15 @@
 
 CMutableTransaction ConstructTransaction(const UniValue& inputs_in, const UniValue& outputs_in, const UniValue& locktime, bool rbf)
 {
-    if (inputs_in.isNull() || outputs_in.isNull())
+    if (outputs_in.isNull())
-        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, arguments 1 and 2 must be non-null");
+        throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output argument must be non-null");
+
+    UniValue inputs;
+    if (inputs_in.isNull())
+        inputs = UniValue::VARR;
+    else
+        inputs = inputs_in.get_array();
 
-    UniValue inputs = inputs_in.get_array();
     const bool outputs_is_obj = outputs_in.isObject();
     UniValue outputs = outputs_is_obj ? outputs_in.get_obj() : outputs_in.get_array();
 

src/wallet/rpcwallet.cpp

@@ -3997,7 +3997,7 @@ UniValue walletcreatefundedpsbt(const JSONRPCRequest& request)
                 "\nCreates and funds a transaction in the Partially Signed Transaction format.\n"
                 "Implements the Creator and Updater roles.\n",
                 {
-                    {"inputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The inputs. Leave empty to add inputs automatically. See add_inputs option.",
+                    {"inputs", RPCArg::Type::ARR, RPCArg::Optional::OMITTED_NAMED_ARG, "Leave empty to add inputs automatically. See add_inputs option.",
                         {
                             {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                 {

test/functional/rpc_psbt.py

@@ -94,6 +94,9 @@ class PSBTTest(BitcoinTestFramework):
         psbtx1 = self.nodes[0].walletcreatefundedpsbt([{"txid": utxo1['txid'], "vout": utxo1['vout']}], {self.nodes[2].getnewaddress():90}, 0, {"add_inputs": True})['psbt']
         assert_equal(len(self.nodes[0].decodepsbt(psbtx1)['tx']['vin']), 2)
 
+        # Inputs argument can be null
+        self.nodes[0].walletcreatefundedpsbt(None, {self.nodes[2].getnewaddress():10})
+
         # Node 1 should not be able to add anything to it but still return the psbtx same as before
         psbtx = self.nodes[1].walletprocesspsbt(psbtx1)['psbt']
         assert_equal(psbtx1, psbtx)