fix: 404 when last organisation doesn't exist

[rolodato]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

Fix 404 when the last organisation doesn't exist - not exactly sure how this happens but probably involves removing a user from an org.

How did you test this code?

Not tested - I've landed on this route several times in production and it's the only place where it is referenced in code.

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Name	Status	Preview	Comments	Updated (UTC)
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[github-actions]

Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4624	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4624	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4624	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4624	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4624	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4624	Finished ✅	Results ✅

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