Fix "QFSFileEngine::open: No file name specified" warning

The transition repository returns an empty string in
assetPreferredListPath(), which QFile doesn't really like.

Test Plan: Builds

Reviewers: mardelle

Differential Revision:
1 job for master in 1 second (queued for 1 second)
Status Name Job ID Coverage
failed linux kf5-qt5 qt5.10 #10457


Name Stage Failure
linux kf5-qt5 qt5.10 Build
Running with gitlab-runner 11.11.2 (ac2a293c)
on fiesta umZyLAru
Using Docker executor with image kdeorg/ci-suse-qt510 ...
Pulling docker image kdeorg/ci-suse-qt510 ...
ERROR: Job failed: Error response from daemon: pull access denied for kdeorg/ci-suse-qt510, repository does not exist or may require 'docker login' (executor_docker.go:175:1s)