[BUGFIX] Disable extension scanner if no extensions are available

The link to the extension file scanner is removed to avoid any
exception when running a scan if no extension is available at all.

Visually, it's grayed out like the disabled "Update TYPO3 Core"
card in composer mode and the link is removed.

Releases: master, 9.5
Resolves: #90981
Change-Id: I53d41111b51ed74d142894c56a4378fc9d958e8d
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64100


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Benni Mack <benni@typo3.org>