Handle empty multiple_checkbox

[frafra]

Fix #387.

I am not even sure if the "required" check code if ever run, since the validator is not executed when the field is missing when handling package_patch:

ckanext-scheming/ckanext/scheming/validation.py

Lines 152 to 153 in 8646a9d

	if field.get('required') and not selected:
	errors[key].append(_('Select at least one'))

Therefore, I guess this patch should also fix this issue as well, since the validator should always be triggered. I have not tested it, though.