Fix bug #78226: Don't call __set() on uninitialized typed properties

Fix bug #78226: Don't call __set() on uninitialized typed properties

Assigning to an uninitialized typed property will no longer trigger
a call to __set(). However, calls to __set() are still triggered if
the property is explicitly unset().

This gives us both the behavior people generally expect, and still
allows ORMs to do lazy initialization by unsetting properties.

For PHP 8, we should fine a way to forbid unsetting of declared
properties entirely, and provide a different way to achieve lazy
initialization.