conf: fix the device parsing when arguments has no defaults

conf: fix the device parsing when arguments has no defaults

The commit bf528b90 sets the value to an empty string which causes
these errors:

   pcm_hw.c: Invalid type for device
   pcm_hw.c: Invalid type for subdevice

When device arguments (@args) have no default values set,
the field must be skipped to keep the compatibility.

BugLink: https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/issues/1246
Fixes: https://github.com/alsa-project/alsa-lib/issues/199
Fixes: bf528b90 ("conf: add possibility to evaluate simple integer math expressions")
Signed-off-by: Jaroslav Kysela <perex@perex.cz>