strlen(): Passing null to parameter #1 ($string) of type string is deprecated

Hello and many thanks for the great plugin that you have created!

Just a small tip:
It would be better to fix it:
strlen(): Passing null to parameter #1 ($string) of type string is deprecated

It comes when a tag have default:shortcode_attr, but shortcode attribute is not defined.

So, function WPCF7_ContactForm::shortcode_attr() returns null but then this null goes to strlen() in WPCF7_FormTag::get_default_option():

$val = $contact_form->shortcode_attr( $this->name );
if ( strlen( $val ) ) {