Fix detection of whether or not an asset file is "empty"

We can't rely on files/strings for this, since the Locale JsCompiler doesn't use either, but still has content.

src/Asset/LessCompiler.php

@@ -38,6 +38,10 @@ class LessCompiler extends RevisionCompiler
      */
     public function compile()
     {
+        if (! count($this->files) || ! count($this->strings)) {
+            return;
+        }
+        
         ini_set('xdebug.max_nesting_level', 200);
 
         $parser = new Less_Parser([