pyln: Change the default argument to exclude in pay

We must use `None` for default arguments since otherwise they aren't
filtered out when serializing the request. In addition default
arguments to functions are initialized once and, if mutable, could
persist internal changes across function calls.

Changelog-None

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

@@ -990,7 +990,7 @@ class LightningRpc(UnixDomainSocketRpc):
 
     def pay(self, bolt11, msatoshi=None, label=None, riskfactor=None,
             maxfeepercent=None, retry_for=None,
-            maxdelay=None, exemptfee=None, exclude=[]):
+            maxdelay=None, exemptfee=None, exclude=None):
         """
         Send payment specified by {bolt11} with {msatoshi}
         (ignored if {bolt11} has an amount), optional {label}