fix: if an attribute is not required, allow it not be set

When using the API would get "Undefinedarray key" errors on missing
custom attributes, even though those attributes were not required.

Now allow the custom attribute to be missing if not a required custom
attribute.

lib/Application/Attributes/AttributeService.php

@@ -175,6 +175,9 @@ public function Validate($category, $attributeValues, $entityIds = [], $ignoreEm
                 continue;
             }
 
+            if (!$attribute->Required() && !array_key_exists($attribute->Id(), $values)) {
+                    continue;
+
             $value = trim($values[$attribute->Id()]);
             $label = $attribute->Label();