fix: use null coalescing operator for missing 'override' config key

makkus183requested to merge
issue/symfony_mailer-3588265:3588265-symfonymailerupdate20002-fails into 2.x

fix: use null coalescing operator for missing 'override' config key

Config::get() does not accept a default value as a second argument.
When the 'override' key is absent, $settings->get('override', []) silently ignores the fallback and returns null, causing array_filter to operate on null instead of an empty array.

Closes #3588265

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