synchroniser : do not use mutable as default argument

Default arguments are evaluated only once meaning if they are mutables
they are shared between every function invocation.
See the warning here:
https://docs.python.org/3/tutorial/controlflow.html#default-argument-values

So use None instead of [] and replace it by the list in the body of the
function.

src/unisync/synchroniser.py

@@ -190,8 +190,8 @@ class Synchroniser:
                 )
 
     def sync(self, remote_root:str, local_root:str,
-             paths:list=[], ignore:list=[], force:bool=False,
+             paths:list | None=None, ignore:list | None = None, force:bool=False,
-             other:list=[]
+             other:list | None = None
              ) -> None:
         """Performs the synchronisation by calling unison.
 
@@ -217,6 +217,13 @@ class Synchroniser:
                 was interrupted.
                 If this happens propagate the error to unisync.
         """
+        if paths is None:
+            paths = []
+        if ignore is None:
+            ignore = []
+        if other is None:
+            other = []
+
         command = [ "/usr/bin/unison", "-root", remote_root, "-root", local_root ]
         for arg in self.args_bool:
             command.append(f"-{arg}")