Is there such thing as a sun allergy?

Photosensitivity. In addition to hypersensitivity to sunlight (uva and/or uvb light) also consider drug-induced photosensitivity. These include sunscreen creams components, thiazides and furosemide, sulfas, antifungals, among other less common drugs. They cause dermatitis in exposed skin areas and resolve with drug discontinuation.
Kinda sorta. Since no one makes ige antibodies to sunlight there is no true solar allergy. Still the sun may trigger symptoms that mimic allergy. Solar sneezing is a neurologic reflex occurring when susceptible persons step out into bright sunlight. Solar hives are a physical urticaria due to light directly triggering mast cells. The cause of sun poisoning (polymorphous light eruption) is unknown.
Yes. Yes, some people are truly allergic to sunlight. Certain wavelengths of sunlight trigger hives, or welts, in their skin at points of exposure. Certainly there are other forms of sun related problems as well including sun poisoning, heat exhaustion and sunburn.