fix(files_sharing): don't crash CleanupShareTarget when mount info is missing

[bakiburakogun]

Summary

occ maintenance:repair --include-expensive aborts in the "Cleanup share names with false conflicts" step:

Error: Call to a member function getRootId() on null in apps/files_sharing/lib/Repair/CleanupShareTarget.php:128

This happens when a problem share's old target has no entry in the recipient's cached mounts — in our case recipients who had not logged in since the share was created, so oc_mounts had no row for those targets. The array lookup at line 122 yields null and the getRootId() call throws; since it is an Error, the catch (\Exception) below doesn't catch it and the whole repair run dies.

Guard the lookup with ?? null — the same way the generateUniqueTarget() callback a few lines above already does — and skip the mount-cache refresh for that share. The share row itself is already updated by moveShare() at that point, and the mount cache rebuilds on the user's next login.

Fixes #63494

How to test

1. Create user shares whose file_target contains a duplicated conflict suffix (Folder (2) (2))
2. Make sure a recipient has no oc_mounts row for that target (e.g. a user who never logged in since the share was created)
3. Run occ maintenance:repair --include-expensive — without this patch the run aborts at that share; with it the step completes (verified on 34.0.3 over 907 problem shares)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

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