filtering null values by conditions?

[seralf]

Hi

is there any way to avoid mapping NULL values?
At the moment in my test it seems that cells containing '' (empty string) is correctly skipped from the mapping process, while null (or NULL) values seems to be threated as string literals, and then processed.
I looked at rml:condition in the examples, but the statement seems to refer to a function which is deprecated or not yet implemented.

thanks in advance for any suggestion.