OK, let me try again. This time I'm understanding you to mean that you haven't yet set the value of the selectOneMenu and it's still null. And you are asking what the consequences are with the selection list?
The answer is that the control should display with the first element in the selection list selected, but the backing value will remain null until the form is actually posted. At that point the selected item in the list will be used to set the value in the backing bean.
You can also allow for a "no selection" selection. I often do this, where I add an extra selection element to the top of the display list whose data value is "" and whose display value is something like "-------" or "(Please make a selection)".
Incidentally, if you have a selection whose data value is an empty
string, you can force the user to make a selection by setting 'required="true"' on the selectOneMenu element. Validators don't work when no value is supplied, but "required=" takes over for that particular case. Not entirely intuitive, but at least it's consistent.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.