[BUGFIX] Show content elements from hidden pages in preview

Content elements from hidden pages are no longer shown in previews.

Fix this by checking with a strong comparison in enableFields() whether
the $show_hidden parameter is set to the default value.

Fixes: #58148
Related: #57814
Releases: 6.2
Change-Id: I2e20db88a513adb1a4d7599591fd3affdb60f209
Reviewed-on: https://review.typo3.org/30212
Tested-by: Dirk Klimpel
Reviewed-by: Mario Rimann
Reviewed-by: David Hoeckele
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs