Per wikipedia it's a patrilineal system, where women are eligible to succeed to the throne but their children aren't. So there have been empresses, but the imperial line itself is still (ostensibly) an unbroken series of male heirs.

> On some occasions [of a reigning empress], the direct male heir was only a toddler and unable to perform the imperial rituals. In such an instance, his mother, aunt or elder sister, if also of Imperial lineage through her patriline, temporarily took over the throne until the child came to puberty, which was deemed sufficient for a boy's accession. An empress' offspring also did not have claim to the throne from the said maternal lineage, so assigning a female to the throne had the convenient effect of postponing succession disputes.