forms: Fix MultipleBooleanField.to_python when the field value is missing

forms: Fix MultipleBooleanField.to_python when the field value is missing

If you write a test for, say, the bundle form of a patch list, you'd
still have to specify the 'no change' value to other form (e.g. the
multiple update one) fields using MultipleBooleanField or else it'd
raise a ValueError when field.clean() is called as part of
form._get_errors().

Signed-off-by: Guilherme Salgado <guilherme.salgado@linaro.org>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>