UPDATE: This has been fixed in Contact Form 7 – v4.2.2
The issue is that whenever Contact Form 7 generates the form shortcode it doesn’t create a name attribute and it doesn’t seem to fallback to using the title attribute of the shortcode. HTML 5 doesn’t like empty name attributes.
One solution until this is fixed could be to add an attribute to your Contact Form 7 shortcode html_name and give it some kind of string value. This will solve the validation issue. It’s something WPCF7 should handle but it works in the meantime.
[contact-form-7 id="4" title="Contact Form" html_name="Contact Form 1"]