Bug fixes discovered while investigating issues in change facility workflow

[rtibbles]

Summary

• Fixes warning shown because a prop was being passed as a text string true rather than boolean true
• Catches errors raised by trying to save a UserSessionLog for a user that has been deleted by the background mergeuser task

References

Discovered while investigating #12486

Reviewer guidance

This is just a prop fix and a defensive programming check, should be evaluable by code review.

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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[bot]]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.17.0b3.dev0+git.31.g8242f5cf-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0b3.dev0+git.31.g8242f5cf-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0b3.dev0+git.31.g8242f5cf.dmg
Android Package (APK)	kolibri-0.17.0b3.dev0+git.31.g8242f5cf-0.1.3-debug.apk
TAR file	kolibri-0.17.0b3.dev0+git.31.g8242f5cf.tar.gz
WHL file	kolibri-0.17.0b3.dev0+git.31.g8242f5cf-py2.py3-none-any.whl