Fix error with new PHP versions 'Parameter must be an array or an object that implements Countable' (barryvdh#381)

Author: kyrylokulyhin

src/PDF.php

@@ -201,7 +201,7 @@ protected function render(){
 
         if ( $this->showWarnings ) {
             global $_dompdf_warnings;
-            if(count($_dompdf_warnings)){
+            if(!empty($_dompdf_warnings) && count($_dompdf_warnings)){
                 $warnings = '';
                 foreach ($_dompdf_warnings as $msg){
                     $warnings .= $msg . "\n";