Issue backdrop-contrib#48: Add hook_field_insert() to remove empty ta…

…rget_id when there are multiple.

reference.module

@@ -253,6 +253,17 @@ function reference_field_validate($entity_type, $entity, $field, $instance, $lan
   }
 }
 
+/**
+ * Implements hook_field_insert().
+ */
+function reference_field_insert($entity_type, $entity, $field, $instance, $langcode, &$items) {
+  foreach ($items as $delta => $item) {
+    if (reference_field_is_empty($item, $field)) {
+      unset($items[$delta]);
+    }
+  }
+}
+
 /**
  * Implements hook_field_formatter_info() to register the field formatter.
  */