[5.5] return empty string if database session key was expired (#21277)

*           return empty string if database session key was expired

*         return empty string at end of method

src/Illuminate/Session/DatabaseSessionHandler.php

@@ -93,14 +93,16 @@ public function read($sessionId)
         if ($this->expired($session)) {
             $this->exists = true;
 
-            return;
+            return '';
         }
 
         if (isset($session->payload)) {
             $this->exists = true;
 
             return base64_decode($session->payload);
         }
+
+        return '';
     }
 
     /**