fix: Prevent crash when refreshing metadata for files without an owner

[Farcry69]

This crash was mostly happening when using external storages combined with for example the command "occ memories:migrate-google-takeout", or other commands.

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[skjnldsv]

Code make sense

[skjnldsv]

/backport to stable30

[skjnldsv]

/backport to stable29

[skjnldsv]

/backport to stable28

[DaphneMuller]

@skjnldsv can you please check about the the failing tests and ping a second-best reviewer?

[welcome]

Thanks for your first pull request and welcome to the community! Feel free to keep them coming! If you are looking for issues to tackle then have a look at this selection: https://github.com/nextcloud/server/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22