I would eat unless they have told you otherwise. I ate with both my tests.. long and short. Id just stear clear of anything with sugar in it tbh. No juice, no jam etc for breaky. Just have some toast with vegemite and a glass of milk or water.
I ate a biscuit before my first one with DS and had to end up doing the long test as my level was 9.4! Long test came back fine.

Best of luck with it.