Handle missing cached share mounts

[Sudaisib]

Fixes #62545

Summary

CleanupShareTarget can encounter a missing recipient mount-cache entry while repairing a share target. The previous code dereferenced the missing entry before updating the target, causing the repair to fail. This change treats the cached mount as optional and only rewrites the cache when the entry exists, allowing the repair to continue safely.

Validation

• PHP syntax and formatting checks passed locally.
• • The focused database-backed PHPUnit test could not be run in this environment because the required Nextcloud test services were unavailable.

Checklist

• Code is properly formatted
• - [x] Commit includes the required sign-off
• - [ ] Focused regression test will be added to this branch in the next commit

[joshtrichards]

#63495 + tests?

Target issues #63494 and #62545

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