Fix no registration limit setting (#7751)

app/Controllers/configureController.php

@@ -503,7 +503,7 @@ class FreshRSS_configure_Controller extends FreshRSS_ActionController {
 
 		if (Minz_Request::isPost()) {
 			$limits = FreshRSS_Context::systemConf()->limits;
-			$limits['max_registrations'] = Minz_Request::paramInt('max-registrations') ?: 1;
+			$limits['max_registrations'] = Minz_Request::paramIntNull('max-registrations') ?? 1;
 			$limits['max_feeds'] = Minz_Request::paramInt('max-feeds') ?: 16384;
 			$limits['max_categories'] = Minz_Request::paramInt('max-categories') ?: 16384;
 			$limits['cookie_duration'] = Minz_Request::paramInt('cookie-duration') ?: FreshRSS_Auth::DEFAULT_COOKIE_DURATION;