Do not write -1 as size for root folder

In some cases when a scan process is in progress (or not started yet),
the root folder might have subfolder with the size of -1.

Since the quota is calculated based on the root folder size, it is not
good to let that size be overwritten. In such case, the size will be
calculated by ignoring the non-scanned subfolders.

At some point, the scan process in these subfolders will update the root
size again eventually.

Fixes #5509