Optional argumen for GoogleApiServiceClientService::getClient needs to default to NULL (PHP 7.3)

Problem/Motivation

At https://git.drupalcode.org/project/google_api_client/-/blob/8.x-3.x/src/... private function for

::getClient(Google_Client $client ) 

The $client argument is correctly typed to the Google_Client class but the caller allows $client to be a null (also PHPDocs say so). in PHP 7.3 (and probably 7.2 too and that is the reason maybe this has not exploded) that is not allowed and throws an invalid type exception when trying to authenticate a Client.

TypeError: Argument 1 passed to Drupal\google_api_client\Service\GoogleApiClientService::getClient() must be an instance of Google_Client, null given, called in

Steps to reproduce

Deploy the module (via composer) on Drupal 8.9.x under PHP 7.3. Create a Client Entity. try to Authenticate

You should see:

TypeError: Argument 1 passed to Drupal\google_api_client\Service\GoogleApiClientService::getClient() must be an instance of Google_Client, null given, called in

Proposed resolution

Quite simple
::getClient(Google_Client $client = null ) //give it a default, that way it is correctly type but also allows null.

Remaining tasks

Maintainer testing/reproduceability?

User interface changes

None

API changes

None

Data model changes

None