vfs: fix utimes() to handle NULL as argument

If NULL is passed as argument to utimes(), it fails.
man utimes says it should also handle NULL as argument
and update timestamps to current time in this case.

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
tests: add a test for utimes()

Add a small test to check whether utimes() can handle NULL.

Signed-off-by: Jaspal Singh Dhillon <jaspal.iiith@gmail.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>