Fix WP user first/last name being cleared after subscription management form submit

lib/Models/Subscriber.php

@@ -491,6 +491,7 @@ class Subscriber extends Model {
       unset($data['segments']);
     }
 
+    if($subscriber === false) {
       // fields that must exist
       $not_null_fields = array(
         'first_name' => '',
@@ -501,6 +502,7 @@ class Subscriber extends Model {
           $data[$field] = $value;
         }
       }
+    }
 
     // custom fields
     $custom_fields = array();