invoice: "--" means no lunch

Change-Id: I2f42826f9077930841bd77d1cbb2b27209f68c02
diff --git a/invoice.php b/invoice.php
index d262a97..e5e6f19 100644
--- a/invoice.php
+++ b/invoice.php
@@ -43,21 +43,21 @@
     $lunch_count = 0;
     $lunch_details = "";
     $i = 0;
-    if ($user->lunch_day_1) {
+    if ($user->lunch_day_1 && $user->lunch_day_1 != "--") {
         if ($i > 0)
             $lunch_details .= ", ";
         $lunch_count++;
         $lunch_details .= "Day 1: " . $user->lunch_day_1;
         $i++;
     }
-    if ($user->lunch_day_2) {
+    if ($user->lunch_day_2 && $user->lunch_day_2 != "--") {
         if ($i > 0)
             $lunch_details .= ", ";
         $lunch_count++;
         $lunch_details .= "Day 2: " . $user->lunch_day_2;
         $i++;
     }
-    if ($user->lunch_day_3) {
+    if ($user->lunch_day_3 && $user->lunch_day_3 != "--") {
         if ($i > 0)
             $lunch_details .= ", ";
         $lunch_count++;