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

ext/spl/tests/ArrayObject_overloaded_SplFixedArray.phpt

@@ -0,0 +1,28 @@
+--TEST--
+SplFixedArray properties is compatible with ArrayObject
+--FILE--
+<?php
+$ao = new ArrayObject([1, 2, 3]);
+$fixedArray = new SplFixedArray(1);
+$fixedArray[0] = new SplDoublyLinkedList();
+$ao->exchangeArray($fixedArray);
+$ao[0] = new stdClass();
+var_dump($ao);
+?>
+--EXPECT--
+object(ArrayObject)#1 (1) {
+  ["storage":"ArrayObject":private]=>
+  object(SplFixedArray)#2 (2) {
+    [0]=>
+    object(SplDoublyLinkedList)#3 (2) {
+      ["flags":"SplDoublyLinkedList":private]=>
+      int(0)
+      ["dllist":"SplDoublyLinkedList":private]=>
+      array(0) {
+      }
+    }
+    ["0"]=>
+    object(stdClass)#4 (0) {
+    }
+  }
+}