Improve condition in lessphp lib

Library lessphp ( http://leafo.net/lessphp ) version v0.5.0 caused mautic to be flooded with log
messages as described here: (
https://forum.mautic.org/t/log-flooded-with-mautic-notice-php-notice-trying-to-access-array-offset-on-value-of-type-bool-redlining-server/19320/6
), to get rid off the log messages we have strengthen the condition on line that was causing log
messages.

plugins/MauticFocusBundle/Include/lessc.inc.php

@@ -716,7 +716,7 @@ protected function zipSetArgs($args, $orderedValues, $keywordValues)
 
         // check for a rest
         $last = end($args);
-        if ('rest' == $last[0]) {
+        if (is_array($last) && count($last) && 'rest' == $last[0]) {
             $rest = array_slice($orderedValues, count($args) - 1);
             $this->set($last[1], $this->reduce(['list', ' ', $rest]));
         }