installer: Fix logging output

Fix for commit 201e2e8a4f2bf88c142e3bb6a419ad61e3eeba31
os.path.exists raises an Exception if it's argument is None.

Change-Id: I83b7ffe6f8232aa6e237dc2d90bbf2239c504f49
Signed-off-by: Fabian Deutsch <fabiand@redhat.com>