Check the settings for the mentions plugin and check privileges settings.

@julian said:

I imagine if href is missing, this.href would be undefined, no? smile

No. 😛

I was surprised that it's been an empty string that got checked instead of undefined too, but it seems I'm to adapted to jQuery ^^

This is because you use the DOM-element rather than jQuery to get the href:
document.createElement('a').href === ''
$('<a></a>').attr('href') === undefined