Less_Parser: Improve parseDirective() docs, minor toCSS sync

* Follows-up 6edad32 (Ibe33cc358f). It seems return null/void
  and empty array have the same effect in the toCSS visitor for
  Directive nodes. Upstream does void, so let's do the same.

* Make it clear that parseDirective can return Import and Media
  nodes as well.

Change-Id: Ida803aaaf86b88012c72eb546e1c6b074c1198a8

lib/Less/Parser.php

@@ -2281,12 +2281,12 @@ private function parseMedia() {
 		}
 	}
 
-	//
-	// A CSS Directive
-	//
-	// @charset "utf-8";
-	//
-	// @see less-2.5.3.js#parsers.directive
+	/**
+	 * A CSS Directive like `@charset "utf-8";`
+	 *
+	 * @return Less_Tree_Import|Less_Tree_Media|Less_Tree_Directive|null
+	 * @see less-2.5.3.js#parsers.directive
+	 */
 	private function parseDirective() {
 		if ( !$this->peekChar( '@' ) ) {
 			return;

lib/Less/Visitor/toCSS.php

@@ -54,7 +54,6 @@ public function visitMedia( $mediaNode, &$visitDeeper ) {
 
 	public function visitDirective( $directiveNode, &$visitDeeper ) {
 		if ( $directiveNode->name === '@charset' ) {
-
 			if ( !$directiveNode->getIsReferenced() ) {
 				return;
 			}
@@ -69,7 +68,7 @@ public function visitDirective( $directiveNode, &$visitDeeper ) {
 				//	return $this->visit($comment);
 				//}
 
-				return [];
+				return;
 			}
 			$this->charset = true;
 		}
@@ -86,7 +85,7 @@ public function visitDirective( $directiveNode, &$visitDeeper ) {
 			}
 
 			if ( !$directiveNode->rules ) {
-				return [];
+				return;
 			}
 			if ( $this->hasVisibleChild( $directiveNode ) ) {
 				// marking as referenced in case the directive is stored inside another directive