Fix ReflectionProperty::isInitialized() for hooked props

In zend_std_has_property with ZEND_PROPERTY_EXISTS, we'd just return true when
no get hook was present. However, this function is

Fix ReflectionProperty::isInitialized() for hooked props

In zend_std_has_property with ZEND_PROPERTY_EXISTS, we'd just return true when
no get hook was present. However, this function is supposed to return false for
uninitialized properties. PROPERTY_EXISTS is somewhat of a misnomer. Virtual
properties continue to always return true, given there's no backing value to
check.

Fixes GH-15694
Closes GH-15822

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