Fix exception on PHP 7.1

Author: paragonie-scott

src/Matchers/L33tMatch.php

@@ -49,7 +49,7 @@ public static function match($password, array $userInputs = array())
             foreach ($results as $result) {
                 // Set substituted elements.
                 $result['sub'] = array();
-                $result['sub_display'] = '';
+                $result['sub_display'] = [];
                 foreach ($indexSubs as $i => $t) {
                     $result['sub'][$password[$i]] = $t;
                     $result['sub_display'][] = "$password[$i] -> $t";
@@ -190,4 +190,4 @@ protected static function getSubstitutions($password)
 
         return $map;
     }
-}
+}