Fixes #7686: FormAction/sendmail: uninitialised sendMailAttachment

extension/Source/core/form/FormAction.php

@@ -267,7 +267,7 @@ class FormAction {
         $args[] = SENDMAIL_TOKEN_BODY_MODE . PARAM_TOKEN_DELIMITER . $this->evaluate->parse($feSpecAction[FE_SENDMAIL_BODY_MODE]);
         $args[] = SENDMAIL_TOKEN_BODY_HTML_ENTITY . PARAM_TOKEN_DELIMITER . $this->evaluate->parse($feSpecAction[FE_SENDMAIL_BODY_HTML_ENTITY]);
         $args[] = SENDMAIL_TOKEN_SUBJECT_HTML_ENTITY . PARAM_TOKEN_DELIMITER . $this->evaluate->parse($feSpecAction[FE_SENDMAIL_SUBJECT_HTML_ENTITY]);
-        $args[] = $this->evaluate->parse($feSpecAction[FE_SENDMAIL_ATTACHMENT]);
+        $args[] = $this->evaluate->parse($feSpecAction[FE_SENDMAIL_ATTACHMENT]??'');
 
         // Mail: send
         $sendMail = new SendMail();