@nicolas-grekas why it was merged to 5.4 branch?

Traits/RedisTrait.php(188)

Old code:

$port = $hosts[0]['port'] ?? null;

New code:

$port = $hosts[0]['port'] ?? 6379;

With DSN "redis:///var/run/redis/redis.sock" raise an error:

Redis connection "redis:///var/run/redis/redis.sock?dbindex=5" failed: php_network_getaddresses: getaddrinfo failed: Name or service not known

Because phpredis doesn't allow socket connections with a port

(new Redis)->connect('/var/run/redis/redis.sock', 6379);

Error

PHP Warning:  Redis::connect(): php_network_getaddresses: getaddrinfo failed: Name or service not known in /root/test_redis.php on line 5
PHP Fatal error:  Uncaught RedisException: php_network_getaddresses: getaddrinfo failed: Name or service not known in /root/test_redis.php:5
Stack trace:
#0 /root/test_redis.php(5): Redis->connect()
#1 {main}
  thrown in /root/test_redis.php on line 5

Please rollback it

Please open an issue or better: a PR. Note that a simple rollback is certainly not the solution since this PR fixes a deprecation (argument is not nullable)