qemu: Allow empty script path to <interface/>

qemu: Allow empty script path to <interface/>

Before 9c17d665fdc5f (v1.3.2 - I know, right?) it was possible to
have the following interface configuration:

  <interface type='ethernet'/>
    <script path=''/>
  </interface>

This resulted in -netdev tap,script=,.. Fortunately, qemu helped
us to get away with this as it just ignored the empty script
path. However, after the commit mentioned above it's libvirtd
who is executing the script. Unfortunately without special
case-ing empty script path.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>