For most functions, passing nil for an &optional parameter is the same
as omitting it.
If MARKER is not specified, the new marker does not point anywhere.
It's about the return value.
A marker pointing nowhere, might it be considered valid?
What do you mean by "valid"? Yes, it's a marker. No, it
doesn't have a buffer or position. The same is true of
other Emacs-Lisp objects, such as overlays: they can exist
without having non-nil values for certain of their properties.
Expected such returning nil.
Why? The doc string is pretty clear that it returns a
marker. And it is clear what happens if arg MARKER is
nil: "the new marker does not point anywhere".