Fixed #4485 -- Allow nullable DecimalFields to store NULLs.

                _("This value must be a decimal number."))

    def _format(self, value):

        if isinstance(value, basestring):

        if isinstance(value, basestring) or value is None:

            return value

        else:

            return self.format_number(value)

        return u"%.*f" % (self.decimal_places, value)

    def get_db_prep_save(self, value):

        if value is not None:

        value = self._format(value)

        return super(DecimalField, self).get_db_prep_save(value)

Traceback (most recent call last):

...

ValidationError: [u'This value must be a decimal number.']

>>> f = DecimalField(max_digits=5, decimal_places=1)

>>> x = f.to_python(2)

>>> y = f.to_python('2.6')

>>> f.get_db_prep_save(x)

u'2.0'

>>> f.get_db_prep_save(y)

u'2.6'

>>> f.get_db_prep_save(None)

>>> f.get_db_prep_lookup('exact', x)

[u'2.0']

>>> f.get_db_prep_lookup('exact', y)

[u'2.6']

>>> f.get_db_prep_lookup('exact', None)

[None]

"""