In such a situation , when there is nothing with u to give her as a present..u need to express the love u have upon her in ur words and actions.Remind her the very first moment u have met her in the past , how u felt at that time towards her and how ur friendship changed ur opinion or made it stronger later on...Tell how her friendship with u turned so fruitful and lovely...Even tell her the reason for not buying any gift on her birthday...She'll truly get convinced...!!
Otherwise, just pendown ur thoughts in a paper and give it to her as a birthday greeting..!You may even hug or kiss her to express ur love towards her..!!! All the best..!! May ur friendship live long..!!!