[ext.pages] Fix logic for page_count calculation

[krittick]

Summary

This fixes an issue introduced in #1227 with the logic for determining the value for page_count in Paginator.__init__(). It also adds the same fix to Paginator.update().

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)