fix(dav): Add fallback for owner of dav nodes on public shares

[susnux]

• Resolves [Bug]: The FilePicker does not load for moving or copying a file or a folder when not logged in nextcloud/server#45420

[codecov]

Bundle Report

Changes will increase total bundle size by 398 bytes ⬆️

Bundle name	Size	Change
@nextcloud/files-esm	105.61kB	199 bytes ⬆️
@nextcloud/files-cjs	106.55kB	199 bytes ⬆️

[skjnldsv]

Wouldn't have this been better to have getCurrentUser returns anonymous translated values?

{
	uid: 'anonymous',
	displayName: t('Anonymous account'),
	isAdmin: false,
}

[susnux]

I do not think so, because it is meant to return a user, but in this case it is no real user.
So return null is fine.

Otherwise you can have an issue if you have a real user called anonymous