I think even if it was a child's nightlight, it still would not matter. I don't think a few fun bits and pieces from childhood mean you can't function as an adult!

I think this may be one of those situations where you realise it's ok for you and your therapist to not have the same opinions on something, ie having the night light at all. And like you mention, at some point it would probably be useful to delve into the upsetting connotations pitch darkness has for you.