Documentation of listsendpays

[Lagrang3]

According to the documentation of listsendpay the field amount_msat is optional:

    amount_msat (msat, optional): The amount delivered to destination (if known)

No further information is provided. Can the documentation be more explicit? Like explaining, maybe, under which circumstances this field is not present, so that people (and software) calling listsendpays can deduce what is happening with that sendpay and act accordingly.