When no iterations are present show empty state

What does this MR do and why?

When no iterations are found we present the empty state svg with a link to create an iteration.

Resolves #349788 (closed)

Screenshots or screen recordings

How to set up and validate locally

• Feature.disable(:iteration_cadences)
• Go to a project -> iterations
• Make sure there are no iterations present

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

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.