JQuery :: Null - Undefined - Empty - Drop Down Option Is Selected In A Drop Down List ?

I am asking jQuery to tell me which drop down option is selected in a drop down list - like this:

I would like to check if this was successful before I proceed. What are the possible return values for this statement?

If no id exists.
If no option is selected.
If some other problem occurred.

In these cases am I expecting a null return; an undefined return, a false return value?

And, based upon the complete set of return possibilities, what would be the best and most comprehensive tests I could apply to cover every base.