Resolve "localStorage.clear() error when Capybara test fails before visiting page"

This change prevents localStorage.clear() from running when no page has been visited in a Capybara test. This will prevent confusion when unrelated test failures occur before visiting pages in our feature tests.

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I've run this patch against EE which is currently failing with one of these errors:

Before:

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/54119553

Failures:

  1) Subgroup Issuables shows the full subgroup title when issues index page is empty
     Got 0 failures and 2 other errors:

     1.1) Failure/Error: let!(:group)    { create(:group, name: 'group') }

          ActiveRecord::RecordInvalid:
            Validation failed: Path has already been taken
          # ./spec/features/projects/sub_group_issuables_spec.rb:4:in `block (2 levels) in <top (required)>'
          # ./spec/spec_helper.rb:196:in `block (2 levels) in <top (required)>'

     1.2) Failure/Error: execute_script("localStorage.clear();")

          Selenium::WebDriver::Error::NoScriptResultError:
            <unknown>: Failed to read the 'localStorage' property from 'Window': Access is denied for this document.
              (Session info: headless chrome=63.0.3239.108)
              (Driver info: chromedriver=2.33.506092 (733a02544d189eeb751fe0d7ddca79a0ee28cce4),platform=Linux 4.13.5-coreos-r2 x86_64)
          # ./spec/support/capybara.rb:82:in `block (2 levels) in <top (required)>'
          # ./spec/spec_helper.rb:196:in `block (2 levels) in <top (required)>'

After:

https://gitlab.com/gitlab-org/gitlab-ee/-/jobs/54126736

  1) Subgroup Issuables shows the full subgroup title when issues index page is empty
     Failure/Error: let!(:group)    { create(:group, name: 'group') }

     ActiveRecord::RecordInvalid:
       Validation failed: Path has already been taken
     # ./spec/features/projects/sub_group_issuables_spec.rb:4:in `block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:196:in `block (2 levels) in <top (required)>'

Closes #43570 (closed) Related to: gitlab-org/gitlab-ce#40677