Symfony version(s) affected: >= 4.4

Description
A php notice appears in DotEnv when dealing with empty default env variables:
PHP Notice: Uninitialized string offset: 1 in /vendor/symfony/dotenv/Dotenv.php on line 477

if ('' === $value && isset($matches['default_value'])) {
    $unsupportedChars = strpbrk($matches['default_value'], '\'"{$');
    if (false !== $unsupportedChars) {
         throw $this->createFormatException(sprintf('Unsupported character "%s" found in the default value of variable "$%s".', $unsupportedChars[0], $name));
    }

    $value = substr($matches['default_value'], 2);

    if ('=' === $matches['default_value'][1]) { // Here is the notice
        $this->values[$name] = $value;
    }
}

The issue is that $matches['default_value'] is equal to an empty string, not an array.

In my case:

Array
(
    [0] => ${RABBITMQ_USER}
    [backslashes] => 
    [1] => 
    [opening_brace] => {
    [2] => {
    [name] => RABBITMQ_USER
    [3] => RABBITMQ_USER
    [default_value] => 
    [4] => 
    [closing_brace] => }
    [5] => }
)

It's maybe due to how we wrote our env variable. The issue happens when resolving MESSENGER_DSN:

RABBITMQ_USER=guest
RABBITMQ_PASSWORD=guest
RABBITMQ_HOST=127.0.0.1

MESSENGER_DSN=amqp://${RABBITMQ_USER}:${RABBITMQ_PASSWORD}@${RABBITMQ_HOST}:5672/%2f/queue

Possible Resolution

I'm not sure I'm allowed to use such notation for an env variable.
So maybe we should check that $matches['default_value'] is an array and throw an exception if not (and update the documentation?).
Or it could be an empty string and in this case, we should verify the $matches['default_value'] type before going further.

I will be more than happy to do a PR for that, I just need more insight on what to do