[BUGFIX] Do not try to fetch additionalFields for invalid tasks

A task may be invalid when editing so there is no instance of
AbstractTask in $task. With this patch the SchedulerModuleController no
longer tries to add the additionalFields for that as the
AdditionalFieldProviders expect a valid task object when being called on
edit.

Additionally some cleanup in the PHPDoc of AdditionalFieldProviders is
made to make it clear the given task may be NULL on create.

Resolves: #79141
Releases: master, 8.7
Change-Id: Ia7cf8173e65cfdede89a776cff7615678e4ffb21
Reviewed-on: https://review.typo3.org/55122


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Alexander Opitz <opitz.alexander@googlemail.com>
Tested-by: Alexander Opitz <opitz.alexander@googlemail.com>
Reviewed-by: Benjamin Kluge <b.kluge@neusta.de>
Reviewed-by: Henning Liebe <h.liebe@neusta.de>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>