Allow empty commented lines.

```stylus
// This sort of thing (empty line below
// with trailing-space trimmed)
//
// ...shouldn't trigger a warning. Mmmmkay?
```

src/checks/commentSpace.js

@@ -2,7 +2,6 @@
 
 var commentRe = /\/\/ /
 
-
 /**
  * @description // check for space after line comment
  * @returns {boolean} true if comment found, false if not
@@ -16,8 +15,9 @@ var commentSpace = function() {
 	if ( commentRe.test( this.cache.comment ) ) {
 		spaceAfterComment = true
 	}
+	var emptyComment = /\/\/$/.test( this.cache.comment )
 
-	if ( this.state.conf === 'always' && spaceAfterComment === false ) {
+	if ( this.state.conf === 'always' && spaceAfterComment === false && !emptyComment ) {
 		this.msg( 'line comments require a space after //' )
 	}
 	else if ( this.state.conf === 'never' && spaceAfterComment === true ) {