If a person constantly invalidates your feelings is he/she a narcissist?
It really depends. first of all, the problem might be in the person who asks, maybe they really are super sensitive and what they are upset about is not rational.

Also you can never know from someones words that they lack empathy or not.
Some people are just tough, maybe he was in some back breaking situations in life that yours seem minor.

And also lack of empathy is just a small part of narcissism and is not a "only and only" determinant.

If you wanna see if others have empathy or not, you better watch their acts rather than analyzing their words. do they have criminal tendencies or ever were accused of criminal act? are they non-empathetic in their job? are they careless about others life situations? are they reckless? do they put others in trouble to get something they want? and so on.