[9.x] replace string|null with ?string

[PerryvanderMeer]

Not sure if we want to go this way, but I'll give it a try.
Currently, string|null and ?string are used through eachother. For example:

framework/src/Illuminate/Auth/Access/HandlesAuthorization.php

Line 22 in 37bbb8c

* @param string|null $message

framework/src/Illuminate/Auth/Access/HandlesAuthorization.php

Line 35 in 37bbb8c

* @param ?string $message

I'd prefer ?string for variables with a single type, since it improves readability.
This PR replaces string|null with ?string and corrects those internal inconsistencies.
If this PR gets merged, I'll work on the other types.

[ollieread]

I'd prefer ?string for variables with a single type, since it improves readability. This PR replaces string|null with ?string and corrects those internal inconsistencies. If this PR gets merged, I'll work on the other types.

I believe the general convention is to use the shorthand of ?string for typing within the code itself, but to use a union type in documentation. This isn't a Laravel-specific thing, but a general PHP thing.

[dammy001]

@PerryvanderMeer These changes are not needed since doctypes have been removed in L10

[driesvints]

@dammy001 not exactly. They're only removed on stubs which are published as userland code. Any internal code keeps their docblocks.

[driesvints]

@PerryvanderMeer I've sent in a PR to do the opposite: #45677

In general it's good to assume that the least change needed is the way to go. Thanks

[PerryvanderMeer]

@driesvints In the end that seems to be the best solution. Thanks!