Make inspecting SplFixedArray instances more memory efficient/consistent, change print_r null props handling (#9757)

* Make handling of SplFixedArray properties more consistent

Crea

Make inspecting SplFixedArray instances more memory efficient/consistent, change print_r null props handling (#9757)

* Make handling of SplFixedArray properties more consistent

Create a brand new reference counted array every time in SplFixedArray
to be freed by the callers (or return null).
Switch from overriding `get_properties` to overriding `get_properties_for` handler

* Print objects with null hash table like others in print_r

Noticed when working on subsequent commits for SplFixedArray.
Make whether zend_get_properties_for returns null or an empty array
invisible to the end user - it would be always be a non-null array for
user-defined classes.
Always print newlines with `\n\s*(\n\s*)` after objects

Noticed when working on SplFixedArray changes, e.g. in
ext/spl/tests/SplFixedArray__construct_param_null.phpt

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