You can eat Guava fruit on an empty stomach but if you do it can increase the production of acid in your stomach.
It's best to eat something else first such as cereal, toast etc before eating the Guava or any other fruit.
Too much fructose and fiber when eaten on an empty stomach can slow down your digestive system making it harder for you to digest the food.
So it's best to eat something else first other than fruit and then once your stomach has some food in it then eat the Guava.