[VarDumper] Don't pass null to parse_url()

Author: derrabus

VarDumper.php

@@ -71,7 +71,7 @@ private static function register(): void
                 $dumper = new CliDumper();
                 break;
             case 'server' === $format:
-            case 'tcp' === parse_url($format, \PHP_URL_SCHEME):
+            case $format && 'tcp' === parse_url($format, \PHP_URL_SCHEME):
                 $host = 'server' === $format ? $_SERVER['VAR_DUMPER_SERVER'] ?? '127.0.0.1:9912' : $format;
                 $dumper = \in_array(\PHP_SAPI, ['cli', 'phpdbg'], true) ? new CliDumper() : new HtmlDumper();
                 $dumper = new ServerDumper($host, $dumper, self::getDefaultContextProviders());