[BUGFIX] Do not show System Extensions Wizard when done

The check if the wizard has been executed before and
includes all available system extensions have been installed
is wrong. Rather than casting to bool, we should
check if the difference between "seen extensions" and
available system extensions is an empty array.

This correctly shows the wizard when not finished and
hides it when finished.

Resolves: #49256
Releases: 6.2, 6.1, 6.0
Change-Id: I7bd655b2b7eeaaa5b1dd91e80564f150b1d0d038
Reviewed-on: https://review.typo3.org/21751
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters