[Information Panel] Remove nullptr checks for MetaDataWidget member

Summary:
The widget has the same livetime as the containing widget, no need to
do any extra checks.

Reviewers: Dolphin, elvisangelaccio

Reviewed By: Dolphin, elvisangelaccio

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D20514