[BUGFIX] Render plugin content element only if a plugin is selected

This patch ensures, that the plugin content element only calls the
f:cObject viewHelper, when a plugin is selected in the list of
available plugins.

Resolves: #92413
Releases: master, 10.4, 9.5
Change-Id: I0fc8de43d5f72b6e113a00ed224c888da1981737
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/65882


Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Guido Schmechel <guido.schmechel@brandung.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>