fix(customer-type): Do not add empty firstname and lastname to xero payload

[ivannovosad]

Context

We currently only create Lago customers as companies, but there is a need to support both companies and individuals. This change is motivated by scenarios where customers may be a mix of B2B and B2C, and where external integrations require handling both Contacts and Companies.

To address this, we are introducing a new field, customer_type, to distinguish whether a customer is a company or an individual. Existing customers will remain unaffected with customer_type set to the default nil.

Description

We should not send empty firstnme and lastname to Xero, because in that case a contact (instead of a customer) is created.
If we don't send those empty attributes at all a customer is created which is the expected behaviour.