Don't chuck it out yet cdd, sometimes plants look like male initially but given a little more time you can tell for sure, plants can form little pods that look like male plants but in fact turn out to be female because they are not developed enough. But like green said there is a good chance it is male, this is just a small chance it will turn out female.
I switched 12/12 today, well I switched the bulb the nutes and the water schedule but it will only get 3 hours extra dark on this schedule then change to 12/12 from tomorrow. No pre-flowers as yet as I just changed so will give it a week. Boosted the nutes up as well so hopefully that wont cause to many issues, plants were looking like they needed more anyway.
Green a hollow stem in many plants is caused by malnutrition or diseases but thankfully in Cannabis and hemp its actually a good thing, often a plant with a hollow stem is bigger or has a good possibility to produce higher yields. Be happy man, it means that plant is probably going to give you a good harvest. The only down side to hollow stems is if you were trying to clone, it is harder to root than with a plant that isn't hollow.