This change addresses the case when the user joins the live stream
on an ad period but the metadata for the ad period is not emitted.
This results in inserting a partial ad group.
In this case the ad group duration is longer than the partial ad
group. If now the partial ad group ends at the period before the
last period of the window (unknown duration), the splitting algorithm
didn't recognize that the ad group already ended and made the last
period wrongly an ad period.
This change handles this edge case by counting the mapped ads in
the partial ad group to detect this situation and stops splitting.
#minor-release
PiperOrigin-RevId: 539102785
cd604e7ead