Galette

Contribution amount is set into contribution if !empty(); and required fields on isset($this->_amount).

When trying to set "0" as contribution value; _amount property is never set; and that silently fails on contribution update, original amount is always kept.

Contribution creation with an amount of "0" should also fail but... Required fields are based on isFee() which currently returns null on creation; therefore _amount is not mandatory on creation; and value is set to NULL in database. When loading such a contribution, database value is casted, therefore NULL becomes 0 \o/