Ignore CSS declaration with empty content attribute

[dekimsey]

I ran into an issue where the CSS parsing would break when the content attribute was empty. This change will make those declaration NOOPs and skip them.

Example erroring declaration:

.fa-borders:before{
  content:"";
  border: #000000 1px solid;
  padding:1px;
  border-radius:3px;
  font-size:95%;
  width:16px;
  height:16px;
  display:block
}