Fix BCAddressField where field is required but there is no value

[alfetopito]

If the field matches above conditions, validation will raise an AttributeError:

... File ".../python2.7/site-packages/django/forms/forms.py", line 287, in _clean_fields value = field.clean(value) File ".../python2.7/site-packages/django_bitcoin/BCAddressField.py", line 21, in clean value = value.strip() AttributeError: 'NoneType' object has no attribute 'strip'