Fix #4995: Don't show Brave News inline ads when there are no feed items

[kylehickinson]

Summary of Changes

This pull request fixes #4995

Submitter Checklist:

• Unit Tests are updated to cover new or changed functionality
• User-facing strings use NSLocalizableString()

Test Plan:

• Test regular feed, ensure ads and feed items are visible
• Install fresh (required to reset ads cap), disable all sources, verify that no inline ads appear
• Add a source that results in more than 10 feed items and verify ads and feed items are visible

Reviewer Checklist:

• Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• Necessary security reviews have taken place.
• Adequate unit test coverage exists to prevent regressions.
• Adequate test plan exists for QA to validate (if applicable).
• Issue and pull request is assigned to a milestone (should happen at merge time).