Update NEWS and UPGRADING for ext/session changes

NEWS

@@ -87,6 +87,10 @@ PHP                                                                        NEWS
   . Fixed bug GH-20217 (ReflectionClass::isIterable() incorrectly returns true
     for classes with property hooks). (alexandre-daubois)
 
+- Session:
+  . Fixed bug 71162 (updateTimestamp never called when session data is empty).
+    (Girgias)
+
 - Soap:
   . Soap::__setCookie() when cookie name is a digit is now not stored and
     represented as a string anymore but a int. (David Carlier)

UPGRADING

@@ -26,6 +26,10 @@ PHP 8.6 UPGRADE NOTES
 - Session:
   . A ValueError is not thrown if $name is a string containing null bytes in
     session_module_name().
+  . session_encode() now returns an empty string instead of false for empty
+    sessions. It only returns false now when the session data could not be
+    encoded. This mainly happens with the default serialization handler
+    if a key contains the pipe | character.
 
 - Standard:
   . Invalid mode values now throw in array_filter() instead of being silently