Fix GH-17198: SplFixedArray assertion failure with get_object_vars

Because the properties table contains both a numeric index and a string
index that map to 0 in a symbol table, this cau

Fix GH-17198: SplFixedArray assertion failure with get_object_vars

Because the properties table contains both a numeric index and a string
index that map to 0 in a symbol table, this causes an assertion failure.
Looking at the manual page of get_object_vars(), it seems that only real
properties must be included. Given that SplFixedArray's elements are not
accessible like properties, they should be excluded. This restores PHP
8.3 behaviour. The reason that this didn't cause problems on 8.3 is
because it used a different handler (get_properties).

Closes GH-17206.

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