[QGIS-Developer] Python expression function

[Siki Zoltan]

Dear Developers,

I found some strange behaviour of Python expression functions if variable 
length parameter list is given (e.g args=-1).
If there is a NULL value among the parameters the function is not called.
See the discussion on
https://gis.stackexchange.com/questions/240470/qgis-expression-function/240480?noredirect=1#comment376061_240480

If it a feature or a bug?

Regards,
Zoltan