feat(payment_terms): Apply nullability for net_payment_term

[D1353L]

Context

The net_payment_term alias column doesn't have an equivalent for some new payment terms ("end_of_month", "day_of_month" etc.). In those cases, the alias should be null.

Description

• Migration to allow null for the net_payment_term in billing_entities and invoices
• GraphQL type updates to allow returning null to the frontend.

NOTE: the model validation kept restricting null and is changed in the following PR #6195. This is for safety when the current PR is released and the following one isn't. So we don't get net_payment_term: null too early, which could break due_date calculations.

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