Clarify whether `offer_amount` can be zero

[vincenzopalazzo]

The specification for offer_amount (type 8) states:

• if a specific minimum offer_amount is required for successful payment:
  • MUST set offer_amount to the amount expected (per item).
• otherwise:
  • MUST NOT set offer_amount

However, there is no explicit validation requirement stating that offer_amount must be non-zero when present.

Questions:

1. Is offer_amount = 0 a valid value when the field is present?
2. Should readers reject an offer with offer_amount = 0?
3. Or is offer_amount = 0 semantically equivalent to omitting the field entirely?

Adding a clarification like "if set, offer_amount MUST be greater than zero" (or explicitly allowing zero) would help implementers handle this edge case consistently.