API reports required variables with a value of zero as missing

When creating an OptionValue via. API v3, a the weight attribute must be specified in order to pass the mandatory fields check. This is correct, as defined in the DAO.

However, the mandatory_fields check uses empty() to evaluate each field in turn. In the case of weight, '0' is valid. However, PHP classes 0 as an empty value.

In such a circumstance, 0 is actually correct and has a semantic meaning.

Proposed patch for this bug is attached. However, I am unsure whether this is sufficient enough to allow robust validation.