[BUGFIX] Re-add parsing for list elements

Due to patch https://review.typo3.org/33613/ the parsing of list
elements get broken. The issue only occurs if you have added the li tag
as external block element manually without any block configuration.
As this used to work in 6.2 this patch re-adds parsing for those tags.

Releases: master, 6.2
Resolves: #65506
Change-Id: I12c93b2f6aa2bdd073c5af8d7e6eff2433ec9030
Reviewed-on: http://review.typo3.org/38681


Reviewed-by: Andreas Fernandez <andreas.fernandez@aspedia.de>
Tested-by: Andreas Fernandez <andreas.fernandez@aspedia.de>