Fix console errors in Minimap component when all nodes are hidden

[richardwestenra]

Description

The minimap component is throwing errors when there are no nodes displayed on the page.

Development notes

• Add tests to ensure that this doesn't happen again (because it's not the first time this regression has been introduced)
• Fix the error

QA notes

Disable all nodes (e.g. by clicking 'Hide all', or disabling each of the 3 node-types)

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

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