Avoid overwrite last ip or user_agent with empty ones

tlaverdure/laravel-echo-server#546
In some cases request doesn'nt has `ipAddress` or `userAgent`, with this change it avoid overwrite the last reported data

Author: PaolaRuby

src/Illuminate/Session/DatabaseSessionHandler.php

@@ -174,6 +174,13 @@ protected function performInsert($sessionId, $payload)
      */
     protected function performUpdate($sessionId, $payload)
     {
+        if (! $payload['ip_address']) {
+            unset($payload['ip_address']);
+        }
+        if (! $payload['user_agent']) {
+            unset($payload['user_agent']);
+        }
+
         return $this->getQuery()->where('id', $sessionId)->update($payload);
     }