filepreviewjob: allow to log the file being previewed

Méven Carrequested to merge
work/meven/preview-log-improv into master

Better handled the empty file case.

So that:

QT_LOGGING_RULES="kf.kio.gui.debug=true" ~/kde/build/dolphin/bin/dolphin

Outputs a filename before a eventual crash and not just errors.

Context: https://discuss.kde.org/t/kioslave-5-closed-unexpectedly/45427/3

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