Is there anything inappropriate in using “so far” (with the meaning “till now”) in a negative sentence? Eg: I have not come across such a thing so far in my life.
Your sentence is perfectly valid.
You invited me into this stupid haunted house, but I haven't seen a single ghost so far.
I said I'd raise your salary when you started working harder, but so far I haven't noticed a difference.
I've never been to Ireland so far, but I am going next September.