Add regression test and fix for empty querysets being passed to get_content_nodes_data

[rtibbles]

Summary

• It is possible with partial metadata import for empty querysets to get generated during our file import queries
• Fixes this by doing a defensive check for the queryset being empty (by checking the aggregations return any value)
• Adds a regression test to ensure this behaviour

References

Fixes #11488

Reviewer guidance

Check the test. See the test pass. Rejoice!

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0b7.dev0_git.2.g2a03a265.pex
Windows Installer (EXE)	kolibri-0.16.0b7.dev0+git.2.g2a03a265-unsigned.exe
Debian Package	kolibri_0.16.0b7.dev0+git.2.g2a03a265-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b7.dev0+git.2.g2a03a265-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b7.dev0+git.2.g2a03a265-0.1.0-debug.apk
TAR file	kolibri-0.16.0b7.dev0+git.2.g2a03a265.tar.gz
WHL file	kolibri-0.16.0b7.dev0+git.2.g2a03a265-py2.py3-none-any.whl