mapper.mean.exclude-nulls has the same behavior as
This mapper function returns the mean of all the values found in each sliding window. The associated location is the centroid of all the encountered locations. The associated elevation is the mean of the encountered elevations.
mapper.mean.exclude-nulls function can only be applied to values of type LONG or DOUBLE, when applied to STRING or BOOLEAN it does not return any value.
If a value is NULL in the bucket, the result is NULL