Properly handle deleted contributor when serializing other locations

[jwalgran]

NOTE: This is targeted at the release/2.21.1 branch

Overview

This change avoids a crash in the FacilityDetailsSerializer when the contributor
of a match with an alternate location has been deleted.

Connects #949

Notes

This should have been part of #950

Testing Instructions

• Checkout the release/2.21.1 branch
• Run ./scripts/resetdb and ./scripts/server
• Browse http://localhost:8081/admin/api/contributor/15/change/ and delete the contributor.
• Browse http://localhost:6543/?q=regina and select the facility in Shenzhen. Verify that an error prevents the details page from loading.
• Checkout this branch, browse http://localhost:6543/?q=regina, and select the Shenzhen facility again. Verify that the facility details load correctly.

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[rajadain]

+1 tested, fixes the bug. Sorry I didn't catch it when reviewing #950.

[jwalgran]

Thanks for the review.

Sorry I didn't catch it when reviewing

No problem. Sorry I didn't think of it when I wrote the initial PR. 👍