Prevented output of empty skip link markup.

[christophherr]

closes #336.

[ibes]

Should there be tests added to check for this behaviour?

[christophherr]

Arguably, keeping the (array) would prevent beans_output_skip_links from ever fataling. The downside is that if you want to shortcircuit the skip links with the filter, you can only use __return_null. Without the casting, you can use __return_false or __return_null in the filter.

I've no strong preference either way.

Yes, tests should be added to beansBuildSkipLinks.php and a few new tests written for beans_output_skip_links