Default argument `None` in kernel functions

My kernel function takes debug_ttl, to which the user is supposed to pass an instance of artiq.coredevice.ttl.TTLInOut. Is there a way to give debug_ttl a default value such as None and to only flip debug_ttl's state if the user passed a smth to it?

In regular python, I would do this, which doesn't work in ARTIQ of course:

@kernel
def function(arg1, arg2, debug_ttl = None):
    [...]
    if debug_ttl != None:
        debug_ttl.off()
    [...]