Taking ibuprofen on an empty stomach can cause some people to experience issues and side effects including nausea, vomiting, diarrhea.
If you take ibuprofen after eating something and have something in your stomach then those side effects which are nausea, vomiting, diarrhea should not occur.
Other than those side effects nothing bad will happen when you do take ibuprofen on an empty stomach but the ibuprofen may also take a bit longer to kick in and work.
So always have something to eat even if it's a piece of toast, a little cereal, bowl of oatmeal etc before taking the ibuprofen.
That way you don't have to worry about experiencing those potential side effects.