PHP deprecated message

Hello and many thanks for the plugin.

Unfortunately, it happens so, if [tag name default:shortcode_atts] but sortcode_atts with name is not defined, it comes to:
strlen(): Passing null to parameter #1 ($string) of type string is deprecated
wp-content/plugins/contact-form-7/includes/form-tag.php:413

It seems that JS-style of the checking the existence of the value doesn’t work for php:
$val = $contact_form->shortcode_attr( $this->name );
this case function returns nothing (or null)
if ( strlen( $val ) ) {
and strlen (null) is depreciated.

Could you fix it, please?