Fix guard for unitialized variable

The original guard was preventing an id with value of
zero from being processed correctly.

site-generator

@@ -290,11 +290,8 @@ sub parse_page_arg {
     # Split page name from page ids if available.
     my ($page, $ids) = split(/=/, $page_arg);
     my @ids;
+    $ids = "" unless defined $ids;
 
-    if(!$ids) {
-        $ids = "";
-    }
-    else {
     # Parse the page ids and push them onto @ids array
     my @ids_by_comma = split(/\,/, $ids);
     foreach my $id_by_comma (@ids_by_comma) {
@@ -309,7 +306,7 @@ sub parse_page_arg {
             verbose (1, "\nWarning: Page $page id range $id_by_comma could not be parsed.");
         }
     }
-    }
+    
     return ('page' => $page, 'ids' => [@ids]);
 }