Contact Form: fix Uninitialized string notice

Props @iandunn

Submitting a contact form with `WP_DEBUG` enabled and Akismet activated produces this PHP notice on a local install:

`( ! ) Notice: Uninitialized string offset: 1 in plugins/jetpack/modules/contact-form/grunion-contact-form.php on line 318`

That's because `akismet_http_post()` returns an empty string on failure, but the Contact Form is expecting it to always be an array.

Original ticket:
http://plugins.trac.wordpress.org/ticket/2010