Do not show an error in tasks stats if no tasks have run yet

[dekkers]

Changes

When an organization has just been created and no tasks have run yet, we showed an error because we check for if stats and stats is empty. This adds a separate stats_error context variable to distinguish between empty and error.

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Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

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Checklist for QA:

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[underdarknl]

Code looks good to me.

[underdarknl]

Checklist for QA:

• I have checked out this branch, and successfully ran a fresh make reset.
• I confirmed that there are no unintended functional regressions in this branch:
  • I have managed to pass the onboarding flow
  • Objects and Findings are created properly
  • Tasks are created and completed properly
• I confirmed that the PR's advertised feature or hotfix works as intended.

What works:

• Page does not show an error when no jobs have been running.

Maybe we'd like to check the ENV related to our scan interval and see if this is indeed something that should have happened, and hint the user on why not jobs where found. (eg, check the health page, check the katalogus for enabled boefjes, check the indemnifications and objects.