Skip uninitialized/unset typed properties when serializing objects in php 7.4+

[TysonAndre]

php/php-src#5396

Igbinary should be usable as a drop-in replacement for the native serializer.

Observed: igbinary_serialize() throws
Expected: igbinary_serialize() should skip the properties, like serialize()

The addition of typed properties had some edge cases with php applications/libraries use of serialization/unserialization that were worked out in subsequent patch releases