Fix 'SVN Log...' crash while watching history in the unversioned directory. Added checks m_log variable contains anything.

Summary:
This commits adds checks of m_log variable contains anything before using it.
More appropriate is not showing 'SVN Log...' as available action at all, this should go in a further commits.
Anyway checks for a m_log variable is a must.

Test Plan: Run 'SVN Log...' in unversioned directory make sure it doesn't crash any more and showes empty history.

Reviewers: #dolphin, meven, elvisangelaccio

Reviewed By: #dolphin, meven, elvisangelaccio

Differential Revision: https://phabricator.kde.org/D29043