Skip non-existing properties returned by __sleep()

[nicolas-grekas]

Serializing a null for properties that don't exist makes no sense.
Better skip them.

I think this old behavior was due to a pure side effect of the implementation, which has been fixed in 90fea67 and can now be fixed.

[TysonAndre]

This seems like an improvement, and would be consistent with the other PR being worked on to omit uninitialized&unset declared typed properties from __sleep.

If there was a need in an application to serialize a literal null, then __serialize is available in 7.4 as an alternative - I'd assume most application writers that had notices enabled would fix the notices.

It should be noted in UPGRADING afterwards if this gets merged, though.

Would like to hear what @nikic and others think

[nikic]

I agree that this change makes sense. I think it would be safer to land it in master only though, assuming there is no particular urgency to make the change.

[nicolas-grekas]

Works for me, now rebased on master.

[cmb69]

Thanks! Applied as 657f756.