Javadoc
Determines whether or not the null value should be included in the list of choices when the
field's model value is nonnull, and whether or not the null_valid string property (e.g.
"Choose One") should be displayed until a nonnull value is selected.
If set to false, then "Choose One" will be displayed when the value is null. After a value is
selected, and that change is propagated to the underlying model, the user will no longer see
the "Choose One" option, and there will be no way to reselect null as the value.
If set to true, the null string property (the empty string, by default) will always be
displayed as an option, whether or not a nonnull value has ever been selected.
Note that this setting has no effect on validation; in order to guarantee that a value will
be specified on form validation,
#setRequired(boolean). This is because even if
setNullValid() is called with false, the user can fail to provide a value simply by never
activating (i.e. clicking on) the component.