txprepare: don't crash on empty args

add test for empty args check

contrib/pyln-client/pyln/client/lightning.py

@@ -1069,7 +1069,7 @@ class LightningRpc(UnixDomainSocketRpc):
         if 'destination' in kwargs or 'satoshi' in kwargs:
             return self._deprecated_txprepare(*args, **kwargs)
 
-        if not isinstance(args[0], list):
+        if len(args) and not isinstance(args[0], list):
             return self._deprecated_txprepare(*args, **kwargs)
 
         def _txprepare(outputs, feerate=None, minconf=None, utxos=None):

tests/test_wallet.py

@@ -280,7 +280,7 @@ def test_txprepare(node_factory, bitcoind, chainparams):
     bitcoind.generate_block(1)
     wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 10)
 
-    prep = l1.rpc.txprepare([{addr: Millisatoshi(amount * 3 * 1000)}])
+    prep = l1.rpc.txprepare(outputs=[{addr: Millisatoshi(amount * 3 * 1000)}])
     decode = bitcoind.rpc.decoderawtransaction(prep['unsigned_tx'])
     assert decode['txid'] == prep['txid']
     # 4 inputs, 2 outputs (3 if we have a fee output).