Fix segfault on no restart args

Summary:
When an application is crashing through KCrash, but has no restart
argument list, KCrash itself crashes because s_autoRestartCommandLine
is null (when it wants to print the command line to stdout). This
patch fixes that by making sure s_autoRestartCommandLine is always
non-null.

Depends on D29810
Signed-off-by: Jiří Paleček <jpalecek@web.de>

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29814