Bugfix for null user-agent

Author: MGatner

src/Visits.php

@@ -104,7 +104,7 @@ public function record()
 		// add session/server specifics
 		$visit->session_id = $this->session->session_id;
 		$visit->user_id = $this->session->{$this->config->userSource} ?? null;
-		$visit->user_agent = $_SERVER['HTTP_USER_AGENT'] ?? null;
+		$visit->user_agent = $_SERVER['HTTP_USER_AGENT'] ?? '';
 		$visit->ip_address = $_SERVER['REMOTE_ADDR'] ?? null;
 
 		// check for an existing similar record