The Bible doesn’t say that a child was conceived.
The account in 2 Samuel 13 doesn’t seem to miss any details of this terrible incident, so it probably would have also mentioned if Tamar had become pregnant. However, it’s silent about it, which probably means that a child wasn’t conceived.
She “remained desolate in her brothers house” which I took to mean she never married and had children. Back then she would have been considered damaged goods.