allow empty directives. fixes #2377

lib/less/tree/directive.js

@@ -38,7 +38,10 @@ Directive.prototype.genCSS = function (context, output) {
         value.genCSS(context, output);
     }
     if (rules) {
-        this.outputRuleset(context, output, [rules]);
+	    if (rules.type === "Ruleset") {
+		    rules = [rules];
+	    }
+        this.outputRuleset(context, output, rules);
     } else {
         output.add(';');
     }

test/css/css-3.css

@@ -147,3 +147,5 @@ body ^^ .shadow {
   symbols: ‣;
   suffix: " ";
 }
+@-ms-viewport {
+}

test/less/css-3.less

@@ -149,3 +149,6 @@ body ^^ .shadow {
   symbols: ‣;
   suffix: " ";
 }
+@-ms-viewport{
+//width: auto !important;
+}