Odoo report field data is "Undefined" when expected False

It’s not ideal, but you could create a kind of computed copy/dummy field on res.partner that stores a Char version of the Boolean field.

Take this for example where the compute method just checks if the field is True and stores as 't' or 'f' depending on that.

is_customer_string = fields.Char('Is a Customer? (text)', compute='_compute_is_customer_string', store=True)

@api.multi
@api.depends('is_customer')
def _compute_is_customer_string(self):
    for partner in self:
        partner.is_customer_string = 't' if partner.is_customer else 'f'

Documentation on computed fields

^{Note: store=True is probably optional, but if you store the field, then you must use some depends field as a trigger to recompute. If you don't store the computed field, then the depends should be left off.}