Fix incorrectly trying to set value when model column is undefined

Author: murrant

LibreNMS/Discovery/YamlDiscoveryDefinition.php

@@ -150,8 +150,10 @@ public function discover($yaml, $attributes = []): Collection
                 /** @var YamlDiscoveryField $field */
                 foreach ($this->fields as $field) {
                     // fill attributes
-                    $field->calculateValue($yamlItem, $fetchedData, $index, $count);
-                    $modelAttributes[$field->model_column] = $field->value;
+                    if ($field->model_column !== null) {
+                        $field->calculateValue($yamlItem, $fetchedData, $index, $count);
+                        $modelAttributes[$field->model_column] = $field->value;
+                    }
                 }
 
                 // if no pollable oid found, skip this index