PHP 8.4: Fix nullable is deprecated

[mukeshpanchal27]

Follow-up #1056

Address the notice:

PHP Deprecated:  Google\Ads\GoogleAds\Lib\V20\UnaryGoogleAdsExceptionMiddleware::__construct(): Implicitly marking parameter $nextHandler as nullable is deprecated, the explicit nullable type must be used instead in /.../vendor/googleads/google-ads-php/src/Google/Ads/GoogleAds/Lib/V20/UnaryGoogleAdsExceptionMiddleware.php on line 41

PHP Deprecated:  Google\Ads\GoogleAds\Lib\V20\UnaryGoogleAdsExceptionMiddleware::__construct(): Implicitly marking parameter $statusMetadataExtractor as nullable is deprecated, the explicit nullable type must be used instead in /.../vendor/googleads/google-ads-php/src/Google/Ads/GoogleAds/Lib/V20/UnaryGoogleAdsExceptionMiddleware.php on line 41

PHP Deprecated:  Google\Ads\GoogleAds\Lib\GoogleAdsMiddlewareAbstract::__construct(): Implicitly marking parameter $nextHandler as nullable is deprecated, the explicit nullable type must be used instead in /.../vendor/googleads/google-ads-php/src/Google/Ads/GoogleAds/Lib/GoogleAdsMiddlewareAbstract.php on line 38

PHP Deprecated:  Google\Ads\GoogleAds\Lib\V20\UnaryGoogleAdsResponseMetadataCallable::__construct(): Implicitly marking parameter $nextHandler as nullable is deprecated, the explicit nullable type must be used instead in /.../vendor/googleads/google-ads-php/src/Google/Ads/GoogleAds/Lib/V20/UnaryGoogleAdsResponseMetadataCallable.php on line 36

[mukeshpanchal27]

@Raibaz @Brothman @AnashOommen Could one of you please take a look and review this PR when you have a moment?

[vikrampm1]

@Raibaz @Brothman @AnashOommen checking on the above. Can you please help review this PR?

[andrii-trush]

The same for V22 #1117