Skip empty detectors

Without this patch, output had ugly messages

    Usage: grep [OPTION]... PATTERNS [FILE]...
    Try 'grep --help' for more information.

and with the patch, the rule file is more readable
because it omits

-# Logitech Powerplay Mat -
-#---------------------------------------------------------------#
-#---------------------------------------------------------------#

-# Quantum Mechanical Keyboard (QMK) -
-#---------------------------------------------------------------#
-#---------------------------------------------------------------#

scripts/build-udev-rules.sh

@@ -80,7 +80,7 @@ do
     type=$(printf %s "$DATA" | grep @type |  sed -e 's/@type//g' -e 's/^ *//g')
     detectors=$(printf %s "$DATA" | grep @detectors |  sed -e 's/@detectors *//g' -e 's/^ *//g' -e 's/\,/\n/g')
 
-    if [[ $type = USB || $type = Serial ]]; then    #Check that the type is USB
+    if [[ -n "$detectors" ]] && [[ $type = USB || $type = Serial ]]; then    #Check that the type is USB
         ## Iterate over the comma seperated detector function list
         while read -r detector
         do