Fix "Undefined index: woo_projects_nonce" notice

Hey Guys,

WordPress throws undefined index notice when trashing and restoring posts from trash.

[Screenshot](http://d.pr/i/1hSDT)

Per [Smashing Mag](http://www.smashingmagazine.com/2011/10/04/create-custom-post-meta-boxes-wordpress/) and [this guy](http://wordpress.aspcode.net/view/63538464303732726667944/undefined-index-atnonce-in-custom-post-metabox), this is caused by not checking if the key in question ("woo_projects_nonce" here) is set before trying to use it to verify the nonce.

The change here resolves this issue on my local install.

Hope this is helpful.