Don't restore global array variables items with `null` value

[Here](https://github.com/sebastianbergmann/global-state/blob/c85828d1d84f2ac9e62469f5ec185f8cdb200e9d/src/Restorer.php#L135) should be `array_key_exists` instead of `isset`.

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For example, before test I have `$_ENV['MY_VAR'] = null`. In second test element `$_ENV['MY_VAR']` already not exists.