fix(macos): guard against empty ocId items crashing FileProvider

[camilasan]

Summary

Fix for #10701.

Item.itemIdentifier comes straight from metadata.ocId. A row with an empty ocId becomes an item with an empty identifier, and the framework aborts with __FILEPROVIDER_BAD_ITEM_MISSING_IDENTIFIER__. Since ocId is the Realm primary key, one bad row persists and re crashes both the 405 collision path and change enumeration, in a loop that only a database wipe cleared.

Resolves

Three guards so a bad row never reaches the framework:

• addItemMetadata refuses to persist an empty ocId (source).
• Item.storedItem returns nil for an empty identifier (collision path).
• toFileProviderItems skips empty ocId rows (enumeration).
• depth 1 folder reads no longer write empty ocId rows straight to Realm, and purge any left by an earlier build
• dolder and file creation reject an empty ocId from the server instead of returning an item with an empty identifier
  Deletion protection: change reporting skips empty identifiers before didDeleteItems

TODO

• This PR closes the crash only. The credential and XPC reconnection and the empty first password behaviors noted in macOS VFS: FileProviderExt crash loop (__FILEPROVIDER_BAD_ITEM_MISSING_IDENTIFIER__) — 405 on folder create embeds DB-derived collision item without identifier; corrupt records then crash change enumeration #10701 are separate and should be tracked in their own issues.

Checklist

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[camilasan]

/backport to stable-34.0