Notice raised when text metatags are left empty

I was working with the metatags on and the panels module, checked "Enable Metatag configuration" and clicked update and save without entering any metatags. I got a slew of notice level errors. I traced the notices down to this line (#97)

$name = "metatag:" . $options["instance"] . ":" . $this->info["name"];

The error can be removed with this replacement line...

 $name = "metatag:" . (isset($options["instance"]) ? $options["instance"] : '') . ":" . $this->info["name"];

This doesn't necessarily fix the issue of instance not being set when this function is called, but it does suppress the notice error. The behavior of the code will be the same before and after - when PHP is asked to add a null or unset value to a string, it adds an empty value instead.