I work in a Dermatology Hospital..
I deal Mainly with Kidney Transplant patients who use Azathioprine and Prednisolone long term. I deal with them because they tend to get skin problems i.e skin cancers after being on treatment long term.
I myself am on Azathioprine and use SPF 50 from April til October and also wear hats when out in the sun !! also try and avoid the sun from midday til evening !!
Trust me its best to stay out of the sun if possible, use high factor sun block and also wear hats if on Azathioprine or Prednisolone.
It is safe if you take pre-cautions.