[ci skip] Fix str_split NEWS and UPGRADE note

Closes GH-9123.

Author: herndlm

NEWS

@@ -81,7 +81,8 @@ PHP                                                                        NEWS
     iterable. (timwolla)
 
 - Standard:
-  . Fixed empty array returned by str_split on empty input. (Michael Vorisek)
+  . Implemented FR GH-8924 (str_split should return empty array for empty
+    string). (Michael Vorisek)
   . Added ini_parse_quantity function to convert ini quantities shorthand
     notation to int. (Dennis Snell)
   . Enable arc4random_buf for Linux glibc 2.36 and onwards

UPGRADING

@@ -45,7 +45,9 @@ PHP 8.2 UPGRADE NOTES
     stripos, strripos, lcfirst, ucfirst, ucwords, str_ireplace,
     array_change_key_case and sorting with SORT_FLAG_CASE use ASCII case
     conversion.
-  . str_split no longer returns an empty array on empty string.
+  . str_split() returns an empty array for an empty string now. Previously it
+    returned an array with a single empty string entry. mb_str_split() is not
+    affected by this change since it was already behaving like that.
 
 - SPL:
   . The following methods now enforce their signature: