Annotations not showing when wanting to hide label

A similar issue to #528 seems to be happening for filled annotations (with `x` and `x2`), see pen: https://codepen.io/anon/pen/rgdbaZ

Leaving the label text as `undefined` or empty string makes the annotation not render at all, while setting it as a string with one space `' '`, will render it with a label artifact at the top left corner, like #528 
