mdraid/module-setup.sh: fixed print-cmdline for empty UUID

fixes

/usr/lib/dracut/modules.d/90mdraid/module-setup.sh: line 60:
_activated["${UUID}"]: bad array subscript

Thanks to Jan ONDREJ for spotting this!

modules.d/90mdraid/module-setup.sh

@@ -54,6 +54,8 @@ cmdline() {
             done
         )
 
+        [[ -z "$UUID" ]] && continue
+
         if ! [[ ${_activated[${UUID}]} ]]; then
             printf "%s" " rd.md.uuid=${UUID}"
             _activated["${UUID}"]=1