PHP 8.4: fix deprecation notice

[gharlan]

Implicitly marking parameter [...] as nullable is deprecated, the explicit nullable type must be used instead

[mringler]

I want to merge this into perpl, see perplorm#40. Hope that's ok with you, @gharlan?

[gharlan]

Yes, of course!
(Still hope that it gets also merged into the main Propel repo)

[mringler]

This probably should have been merged immediately, it's a really welcome, necessary, uncontroversial fix.
Then again, tests have been broken for over a year now, there are several fixes, like #2007 or #2021, and they don't get merged either.
Adding this was suggested by @oojacoboo, I am waiting for his confirmation and then it'll be part of Perpl.
Thank you for your kind approval, and thank you for the fix!

[smhg]

@gharlan would you be able to rebase your work and update the PR please? This will make the mysql tests run (as they have been fixed). Thanks!

[PhilinTv]

@gharlan now, when the master is green, we can merge it.
thanks for the PR!

@mringler indeed! now it's merged to the main repo.