Fix bad reliance on env variable, causing error when interpolating a comma seperated value

lib/less/tree/value.js

@@ -22,7 +22,7 @@ tree.Value.prototype = {
         for(i = 0; i < this.value.length; i++) {
             this.value[i].genCSS(env, output);
             if (i+1 < this.value.length) {
-                output.add(env.compress ? ',' : ', ');
+                output.add((env && env.compress) ? ',' : ', ');
             }
         }
     },