Make default value of event_variables None

Change the default value of event_variables to None instead of an empty FunctorCollection().

Phys/FunTuple/python/FunTuple/functions.py

@@ -15,6 +15,7 @@ from PyConf.Algorithms import FunTupleBase_Composites
 from PyConf.Algorithms import FunTupleBase_ChargedBasics
 from .common_util import conduct_checks_and_transform
 
+
 def _FunTuple_Base(funtuple_func,
                    name,
                    tuple_name,