Find the nothing-here-block before asserting page content when on explore page of shortcuts_spec.rb

What does this MR do?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/builds/15988400

Failures:

  1) Dashboard shortcuts logged out Navigate to tabs
     Failure/Error: expect(page).to have_content('No public groups')
       expected to find text "No public groups" in "Skip to content Toggle navigation Explore GitLab Go Sign in Explore GitLab Discover projects, groups and snippets. Share your projects with others Trending Most stars All Last updated No projects found"
     # ./spec/features/dashboard/shortcuts_spec.rb:41:in `block (3 levels) in <top (required)>'
     # ./spec/spec_helper.rb:76:in `block (2 levels) in <top (required)>'

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?