[BUGFIX] Outside storages can never be marked as public

There are some edge cases where "is_public"
can never be true within a storage record in the BE,
for instance for storage located outside
the document root or for storage driven by special driver
such as Flickr, ...

Change-Id: Id7ce8006f182c98d7a908c2e6c49e0c66a7c9da4
Releases: 6.2
Resolves: #56730
Reviewed-on: https://review.typo3.org/28224
Reviewed-by: Steffen Ritter
Tested-by: Steffen Ritter
Reviewed-by: Frans Saris
Tested-by: Frans Saris