fix(files_sharing): validate empty file parameter in PublicPreviewController

[psimaker]

Fixes #59229

Summary

The getPreview() method in PublicPreviewController had two input validation issues
that caused internal server errors:

Case A: When requesting a public preview for a folder share without a file parameter,
$node->get('') returns the Folder itself. getPreview() then crashes because it expects
a File instance, not a Folder.

Case B: When requesting a non-existent file with mimeFallback=true, the NotFoundException
from $node->get($file) falls into the catch block where $file->getMimeType() is called.
At that point $file is still the original string parameter, not a Node object, causing a
fatal error.

Changes

• Add early return with 400 Bad Request when $file is empty on folder shares
• Split the try-catch into two blocks: one for node resolution, one for preview generation
• This ensures $file is always a valid Node object when mimeFallback code executes

Test plan

• Added testPreviewFolderEmptyFile — empty file parameter returns 400
• Added testPreviewFolderInvalidFileWithMimeFallback — non-existent file with mimeFallback returns 404
• Added testPreviewFolderValidFileNoPreviewWithMimeFallback — valid file, no preview, mimeFallback redirects to mime icon
• Existing tests still pass (no regressions)

[solracsf]

Thanks but this is duplicate ongoing #59253

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