Thank you for reaching out to us.
While it is customary to urinate prior to immersion, this is not obligatory. This custom is intended to ensure that a woman can be relaxed during
immersion and will not need to contract her muscles to "hold it in."
A substance that is fully enclosed within the body, such as urine in the bladder, is not a
chatzitzah. The immersion in the case you describe is fully valid.
Please let us know if we can be of further assistance.