ZP-955 Warnings in diff backend.

Released under the Affero GNU General Public License (AGPL) version 3.

src/lib/default/diffbackend/diffstate.php

@@ -189,10 +189,17 @@ class DiffState implements IChanges {
                     $changes[] = $change;
                 }
 
+                // @see https://jira.z-hub.io/browse/ZP-955
+                if (isset($old_item['mod']) && isset($item['mod'])) {
                     if ($old_item['mod'] != $item['mod']) {
                         $change["type"] = "change";
                         $changes[] = $change;
                     }
+                }
+                else if (isset($old_item['mod']) || isset($item['mod'])) {
+                    $change["type"] = "change";
+                    $changes[] = $change;
+                }
 
                 // unset in $old, so $old contains only the deleted items
                 unset($old[$id]);