Check if sync item is actually a folder before processing it

[camilasan]

• The rename check was not happening since since it was hitting "Move without permission to rename base file"
• The file was then processed as a new file

Possible workaround for #6549 and #4958

• This fix was the hint for me: #6535

[camilasan]

/backport to stable-3.13

[nextcloud-desktop-bot]

AppImage file: nextcloud-PR-6780-008d5312f560ed86b966c4a92e3d207f984aacbe-x86_64.AppImage

To test this change/fix you can simply download above AppImage file and test it.

Please make sure to quit your existing Nextcloud app and backup your data.

[sonarcloud[bot]]

Quality Gate failed

Failed conditions
E Maintainability Rating on New Code (required ≥ A)
16 New Code Smells (required ≤ 0)

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