Initialize SplFixedArray elements to NULL instead of UNDEF

The SplFixedArray API treats all elements as NULL, even if they
have not been explicitly initialized. Rather than initializing

Initialize SplFixedArray elements to NULL instead of UNDEF

The SplFixedArray API treats all elements as NULL, even if they
have not been explicitly initialized. Rather than initializing
to UNDEF an treating that specially in various circumstances,
directly initialize elements to NULL.

This also fixes an assertion failure in the attached test case.

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