How to handle null return values?

[tobiasschweizer]

In Test Case RMLFNOTC0011-CSV, the result is expected to be null (empty). For the test, I chose grel:string_substring but I am not sure if this method can actually produce a null value. For details, see #53.

[elsdvlee]

Checked with Ben: he finds no agreed-upon grel function (i.e. function on the short list, to be implemented by all engines supporting FNML) that return null. So we suggest to ignore this case for the time being.