If everything else on the test is negative, the presence of nitrates does not indicate you have an infection. Bacteria in the urine will produce an enzyme that changes nitrates (a normal finding in urine) to nitrites. When there are nitrites present, there is a UTI.
It is fairly standard, also, to treat a UTI only if it causes symptoms. If you have no symptoms (such as lower abdominal cramping, bladder spasms, frequency, cloudy or smelly urine) there is nothing to worry about.
Cranberry juice can help prevent UTI, as well as plenty of fluid intake.
Yes, if it said nitrites, then you probably have some bacteria in your bladder, but not enough to show as an infection. Though, it is uncommon for a urine sample to show positive for nitrites and not show anything else at all.
Still, you are not symptomatic. So, if your immune system is normal and you are fairly healthy, eating a nutritious diet, drinking plenty of fluids (and don't forget plain water), cranberry juice, and getting some sunshine everyday will keep you healthy. It wouldn't hurt to take a vitamin/mineral supplement also.