The gemara concludes that even when no hairs are found, we suspect that perhaps they fell out (and she is considered a gedolah regarding biblical stringencies). Previously Rava stated that we only assume that hairs are present with regard to stringencies. That would imply that there is also a reasonable assumption that no hairs grew at all. If so, in the above scenario that nothing was found why don’t we simply interpret that as an indication that indeed there were never any hairs present. It would seem that we are simply confirming the reasonable assumption mentioned by Rava.
Quick Search:
Quick Search Daf Insights:
It appears that the primary determinant of gadlus is years; the hairs are simply required to confirm and to complement the maturity resulting from age. Thus even when no hairs were actually found there is still room to suspect that they were present and then fell out, since she has already reached the proper age.