fix(files_external): ignore unsatisfied optional dependencies

[jplitza]

• Resolves: [Bug]: 'Invalid storage backend "smb"' when only smbclient is installed #54047

Summary

Fix setup of external storage with unsatisfied optional dependencies (which were introduced in #52681)

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[jplitza]

I simply cannot understand how my changes would affect the failing tests ("files_sharing: Note to recipient -> shows an existing note when editing a share" timeout), but re-running the failing jobs twice did not fix them…

[skjnldsv]

@jplitza ignore it, it's very unstable at the moment :)
Thanks for the PR!

[skjnldsv]

/backport to stable31

[skjnldsv]

/backport to stable30

[come-nc]

Looks fine I think but I’d like @icewind1991 to check the logic.

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