Form not sent if a select widget is left empty

Hi all,
I have a problem with a form. I have in it two select widgets. One is mandatory while the second not. So, it could be live empty by an user. Furthermore, the “submit” button has a control such as $form.$invalid.
Now, when I test the form it happens that after have made a selection in the first select widget (because it is mandatory), the button becomes active but the form is not sent (and so my task does not go on) until I select a value also on the second select widget. The strange thing that I saw is:
1. if I change the widget with a simple “text input”, the form is sent even if the field is empty
2. with the second select widget, when I display the form, if I make a selection (so I select a value) and after that I change again the selection to the “empty” value, the form is sent to the application (and the record is write in the DB, of course with a “null” field.
Further details:
a. no mandatory fields are required in the DB or in the process contract
b. I made some test also with another browser (Opera instead of IE) but I found the same issue.
Can somebody explain to me what I am doing wrong?
Thank you,