Add qa_selector for new project button to blank state landing page

What does this MR do and why?

Fixes #409892 (closed) by adding a qa-selector to the blank state landing page for projects. See - #409892 (comment 1412636415)

The test appears to have a flake, as it will work when projects already exist for a user. However if the user has no projects the 'blank state landing page' is instead rendered which has a different layout.

When running a full suite, this flake will only manifest if the spec is the very first to be run in the entire suite, otherwise other tests will have created projects for the test user.
It can be easily reproduced in GDK by deleting all projects for a user, so the blank state landing page is rendered.

This selector is the same as is https://gitlab.com/gitlab-org/gitlab/-/blob/0147ed47a8b4032d381265cbd00ab1a0402f3bb7/app/views/dashboard/_projects_head.html.haml#L10 as both elements should not be rendered together.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Ensure the user in GITLAB_USERNAME has no projects on their account so when they login they see the blank state landing page. See #409892 (closed) for example

$ bundle exec rails console
# Use with caution, will delete all projects in GDK
> Project.delete_all

2. bundle exec rspec qa/specs/features/browser_ui/9_data_stores/project/create_project_spec.rb:32
3. Repeat steps for both an admin and non-admin user

MR acceptance checklist

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