Fix crash on triggering "Reformat Source" without no more file loaded

Summary:
Also disable actions "Reformat Source" & "Reformat Line" if no document
is activated

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D4075