NullBooleanSelect should have empty_label or similar

[benjaoming]

NullBooleanSelect is responsible of making the values 1, 2, and 3 turn into None, True or False. That's why having custom "choices" kwarg is perhaps a bit over the top. Well, it's pretty meaningless to have a utility class if you start passing a choices kwarg that contains half of the code that makes up NullBooleanSelect.

However, I need one small feature from this widget! Namely, that the following...

class NullBooleanSelect(Select):
    """
    A Select Widget intended to be used with NullBooleanField.
    """
    def __init__(self, attrs=None):
        choices = (('1', ugettext_lazy('Unknown')),
                   ('2', ugettext_lazy('Yes')),
                   ('3', ugettext_lazy('No')))
        super(NullBooleanSelect, self).__init__(attrs, choices)

...is changed to:

class NullBooleanSelect(Select):
    """
    A Select Widget intended to be used with NullBooleanField.
    """
    def __init__(self, empty_label=None, attrs=None):
        choices = (('1', empty_label or ugettext_lazy('Unknown')),
                   ('2', ugettext_lazy('Yes')),
                   ('3', ugettext_lazy('No')))
        super(NullBooleanSelect, self).__init__(attrs, choices)

The motivation is that I often leave out labels to have them put as the default first option of the Select. An example use:

class MyForm(forms.Form):
    has_payments = forms.NullBooleanField(
        label="",
        required=False,
        widget=NullBooleanSelect(empty_label=_(u"Has previous payments?"))
        help_text=_(u"Only show subscriptions that have previously been charged"),
    )

Even more preferable, would be to place the empty_label kwarg in NullBooleanField, as that would match the options for ModelChoiceField.