Custom links to the current page break if no class is defined

If the user has custom links to the current route, and those links do not have a class defined, this module throws an error while saving.

TypeError: in_array(): Argument #2 ($haystack) must be of type array, null given in in_array()

A usecase where I ran into this is issue is a block rendering facets. This uses the current route as link, but generates links without a classname.

A simple check before accessing class would fix this.