Fix GH-15137: Unexpected null pointer in Zend/zend_smart_str.h (#15138)

This regressed when I optimized $wholeText. The previous code used xmlStrcat
which implicitly checked for a NULL a

Fix GH-15137: Unexpected null pointer in Zend/zend_smart_str.h (#15138)

This regressed when I optimized $wholeText. The previous code used xmlStrcat
which implicitly checked for a NULL argument, but now it's a direct memcpy
which you shouldn't pass null pointers to, although it won't result in a
crash because memcpy doesn't do anything if the length is 0.

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