Render a dash for null metric cells in GLQL tables

What does this MR do and why?

GLQL tables now render — for a null metric cell instead of an italic "None".

lib/gitlab/database/aggregation/authorization.rb prunes a metric the user may not read, such as creditsUsed, which needs read_agent_artifacts. The metric returns null with no error when other metrics are requested. In table.vue, every null went through FieldPresenter, which picks NullPresenter and renders "None". For a metric, that reads as zero credits. The stat and barList presenters already render — for a null metric.

This MR renders — (the shared NO_VALUE) for null metric cells. Dimension cells are unchanged, because a null dimension such as a session with no project is genuinely "None". It adds jest coverage in spec/frontend/glql/components/presenters/table_spec.js.

Found during DAP Impact UAT. The Adoption tab "Group and project comparison" showed "None" in the Credits column for a Developer on gitlab-org.

References

• Resolves https://gitlab.com/gitlab-org/gitlab/-/work_items/631679
• Related UAT epic: https://gitlab.com/groups/gitlab-org/-/work_items/23429

Screenshots or screen recordings

The Adoption tab comparison table for a Developer, before and after the change.

Before: production data as a Developer.

After: the Credits column shows a dash.

Verified on GDK at master with the patch applied against live production responses: the Credits column renders a dash and the Project column keeps "None" for sessions without a project. spec/frontend/glql/components/presenters/table_spec.js passes with 66 examples including the two new ones.

How to set up and validate locally

1. Sign in as a group member without read_agent_artifacts, for example a Developer.
2. Open the DAP Impact dashboard and select the Adoption tab.
3. Check that the Credits column in "Group and project comparison" shows —.
4. Run yarn jest spec/frontend/glql/components/presenters/table_spec.js.

MR acceptance checklist

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