posix: pthread: allow `oldstate` & `oldtype` to be `NULL`

The POSIX standard doesn't specify if the argument to store
previous state/type in
`pthread_setcancelstate`/`pthread_setcancelstate` can be
`NULL`, but threading implementations in Linux & Apache NuttX
permit the arguments to be `NULL`.

This commit changes Zephyr's implementation to mimic that of
Linux & NuttX, so that user do not get caught off-guard by
NULL pointer dereferencing when porting code over from those
OSes.

Updated test accordingly.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>