Error on every function to access gcs storage

[juliano-aiqfome]

The error is: Unable to write file at location: uploads/driver-b54302ec-c772-11ea-b66f-06f3f99250a0/faces/1407502025021167ab59b6c8da0.png. Trying to access array offset on null.

but it happens when I try to read or update a file too, the error is thrown in the line 98 of google/auth/src/CacheTrait.php, $key = $this->cacheConfig['prefix'] . $key;

it happens because the value lifetime and prefix aren't being set on the credentials files, like google/auth/src/Credentials/ImpersonatedServiceAccountCredentials.php or google/auth/src/Credentials/GCECredentials.php, so the attribute cacheConfig is null on the CacheTrait.php, causing the exception, if I set them manually, it works fine.

the full function is

/**
* @param null|string $key
* @return null|string
*/
private function getFullCacheKey($key)
{
if (is_null($key)) {
return null;
}

    $key = $this->cacheConfig['prefix'] . $key;

    // ensure we do not have illegal characters
    $key = preg_replace('|[^a-zA-Z0-9_\.!]|', '', $key);

    // Hash keys if they exceed $maxKeyLength (defaults to 64)
    if ($this->maxKeyLength && strlen($key) > $this->maxKeyLength) {
        $key = substr(hash('sha256', $key), 0, $this->maxKeyLength);
    }

    return $key;
}

I would say that a possible solution is setting a default value right into the CacheTrait.php attribute, something like this:
private $cacheConfig = [
'lifetime' => 1500,
'prefix' => '',
];

[freekmurze]

I'd be a happy to take a look at a PR that included tests.