Make nullable parameters explicitly nullable

[barryhughes]

For improved compatibility with PHP 8.4, and to reduce the amount of error noise we see when running under that version, this change fixes a number of method signatures where parameters are implicitly nullable. For example:

public function __construct( ActionScheduler_Store $store = null, $batch_size = 20 )

$store must be of type ActionScheduler_Store, but if not specified by the caller it is set to null. This is fine, but as of PHP 8.4 we are encouraged to be explicit about it, so we need:

public function __construct( ?ActionScheduler_Store $store = null, $batch_size = 20 ) {

The problem with the above is that ?Type is only valid from PHP 7.1 onwards, whereas we still support PHP 7.0. We also cannot yet use type unions. So, we have a decision to make:

• Bump the minimum supported version of PHP (feels reasonable if it's a small adjustment, from 7.0 to 7.1).
• Dispense with this change and put up with the warning/error noise.

[barryhughes]

💡 Please note that (assuming we are ultimately happy with this change) we should not merge ahead of #1212, which increases the minimum required PHP version to 7.1. Even so, it felt like there was no particular reason to delay review and testing.

(The test fails you will see all relate to PHP 7.0 ... that version will be removed from the test matrix via the other PR I mentioned.)

[barryhughes]

Thanks for the review. Rebased following the merge of #1212, and will merge this if/once all our checks pass.