Fix #4128: Hide 'Favorites' section in absence of favorites

[dooost]

Summary of Changes

Only create the favorites section in the collectionView in FavoritesViewController if the user actually has favorites

This pull request fixes #4128

Submitter Checklist:

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

Test Plan:

• Open a new tab
• Remove all favorites by long pressing on it
• The "Favorites" header should disappear
• Show/hide search history and verify they appear as expected
• Add/remove text to pasteboard and verify the 'Paste & Go' button appears as expected

Screenshots:

With no favorites:

With favorites:

No favorites, hidden search history - is it okay that the view is essentially empty in this scenario?

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).

[iccub]

Great job, very clean PR :)