Fix stats for playlists without a view count estimate

[absidue]

Playlists like How To Build An App: Everything You Didn't Know You Needed To Know don't have an estimate for the number of videos in the playlist, this caused weird behaviour as ytpl always expected the estimate to be there. As a solution for that issue this pull request fixes the parsing of the stats and sets the estimatedItemCount to -1. This is a breaking change as users are likely relying on this number always being equal to or larger than zero.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[absidue]

Still relevant

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.