MDL-62294 lessphp: Fix for PHP 7.2 compatibility

From upstream commit:
https://github.com/oyejorge/less.php/commit/669acc51817a8da162b5f1b7137e79f0e4acc636

lib/lessphp/Visitor/toCSS.php

@@ -110,7 +110,7 @@ class Less_Visitor_toCSS extends Less_VisitorReplacing{
 
 
 		// Compile rules and rulesets
-		$nodeRuleCnt = count($rulesetNode->rules);
+		$nodeRuleCnt = $rulesetNode->rules?count($rulesetNode->rules):0;
 		for( $i = 0; $i < $nodeRuleCnt; ){
 			$rule = $rulesetNode->rules[$i];