Fix show_related_resources and top_level_meta_include_page_count

[bliof]

problem:

When the paginator is :none and top_level_meta_include_page_count is true
The show_related_resources was crashing as the paginator is nil

solution:

Just fixing the immediate problem but the code still leaves page_count
as nil in the meta

Reviewer Checklist:

• Maintains compliance with JSON:API
• Adequate test coverage exists to prevent regressions