[Bug]: Improper input validation in PublicPreviewController triggers internal server error

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

An incomplete input validation in PublicPreviewController can trigger an internal server error.

Steps to reproduce

Case A

1. Create a public link for a folder
2. Send GET https://server33.internal/index.php/apps/files_sharing/publicpreview/{token}
3. 💥

https://github.com/nextcloud/server/blob/7e9e1269a059ddfc7807f977707a1800e3a303e4/apps/files_sharing/lib/Controller/PublicPreviewController.php#L123-L130

• Default for $file is an empty string.
• $file = $node->get(''); is still an Folder instance
• getPreview expectes File

Case B

1. Create a public link for a folder
2. Send GET https://server33.internal/index.php/apps/files_sharing/publicpreview/{token}?file=notexist.png&mimeFallback=1
3. 💥

https://github.com/nextcloud/server/blob/7e9e1269a059ddfc7807f977707a1800e3a303e4/apps/files_sharing/lib/Controller/PublicPreviewController.php#L122-L142

• get and getPreview both throw NotFoundException.
• However the branch with mimetype fallback only works if the preview not exists, not if the node not exists.

Expected behavior

No internal server error