[SWP-5671] Support null AWS credentials for assumed IAM roles

[clemblanco]

When environment variables are not set for example:

'sns' => [
    'key' => env('AWS_ACCESS_KEY_ID'),
    'secret' => env('AWS_SECRET_ACCESS_KEY'),
    'region' => env('AWS_DEFAULT_REGION', 'us-east-1'),
],

which gives

'sns' => [
    'key' => null,
    'secret' => null,
    'region' => 'us-east-1',
],

we should not try to pass an array of empty credentials to the AWS Client like

'credentials' => [
    'key' => null,
    'secret' => null,
],
'region' => 'eu-west-1',

but instead should completely omit the credentials like so

'region' => 'eu-west-1',

This is so we can leverage EC2 Assumed IAM Roles to automatically authenticate with IAM Roles if needed.