I have friends from Spain, Croatia and Romania with this name and they all pronounce it AHNN-na. A bit hard to explain, the first syllable is short, I guess kind of like AHNN (not
ANN). It sounds like
AH but is directly connected with the N. AHNN-na. I have never heard it as AH-na, but maybe that pronunciation is used in some other countries, it would be interesting to know in which ones.