Fix dashboard default variable

[Harriethw]

Description

This was causing pipeline to fail because the type mismatch between object and string

Context

(https://dev.azure.com/nhse-dtos/dtos-manage-breast-screening/_build/results?buildId=30697&view=logs&j=4a575bb6-72e3-5024-4059-681dfff5b130&t=50921939-654e-54f7-a1fc-3563cecb680a&l=170)

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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