[BUGFIX] about - inject object manager only once

Additionally the div containing credits for developers
of loaded extensions is not rendered anymore if no
extensions are loaded.

Change-Id: I00f748281ef6d435f00996b228c9e3583babcdc5
Resolves: #72440
Releases: master,7.6
Reviewed-on: https://review.typo3.org/45441


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>