It's much better to eat something before eating Guava since eating Guava on an empty stomach can increase acid production which is not good.
But it's certainly not gonna harm you if you eat some Guava on an empty stomach but you'll feel much better eating something else first and then eat the Guava.
Eating Guava or other fruits on an empty stomach can slow down your digestive system making it harder for your stomach to digest food as fast as it should.