It's pretty much sterile if you have a catheter. That's why the joke in Dodge ball about Patches drinking his own piss works so well. He's a cripple, so he has a catheter, so "it's sterile and I like the taste!".
At the risk of being the "ackshally" guy, even indwelling catheters substantially increase the likelihood of bacteriuria, or abnormal bacteria in the urine. It gives motile bacteria additional surface area on which to make their way up the ureter, plus the initial placement can introduce microbes.
Bladders are pretty well colonized with a plethora of microbes. Short of nuking it with bleach, it's never going to be sterile.
yeah, that myth is constantly popping up. it's just that whatever bacteria live in your bladder just dont grow outside of it, and thats what makes it look "sterile" if you just check with standard media.
Thats also an issue with everything growing in the stomach, like heliobacter pylori - you have to provide the living conditions of the inside of the stomach to grow and keep a sample for research.