fix: skip empty modalias files in drm module setup

fix: skip empty modalias files in drm module setup

on my system (ThinkPad X201s) I have several modalias entries that are
empty:

    /sys/bus/platform/devices/dock.0/modalias
    /sys/bus/platform/devices/dock.1/modalias
    /sys/bus/platform/devices/dock.2/modalias

executing the drm module setup thus results in the following errors

    dracut-install: No SOURCE argument given

when calling dracut_instmods and hostonly mode is enabled.

Skip those entries, as there are no modules to load in this case
anyways.

Note: it is not sufficient to use [['s -s test (file size is greater
than zero), as all those files are 4096B if you stat them.