Dashboard: When a dashboard has 0 measures/dimensions, show a CTA

[ericpgreen2]

Describe the bug With the "View As" feature, it's possible that a user will see a blank dashboard. For example, with this access policy...

security:
  access: "{{ .user.admin }} OR '{{ .user.domain }}' == 'rilldata.com'"
  include:
    - if: "{{ .user.admin }}"
      names: 
        - total_records

... any non-admin user with the rilldata.com domain will see a dashboard without any measures/dimensions.

Expected behavior Rather than showing a broken dashboard, we should have a CTA that tells the user the problem.