mark values for 'occurrence' and 'unique (appr.)' as 'n/a' for upper paths

[zazi]

currently, those values will be assigned to '0' - albeit, the analysis as it is done right now for this case is wrong (i.e. there are no *.keyword fields for upper paths available). hence, it might be better to write 'n/a' into these cells. furthermore, calculating the occurrence and or cardinality for those upper paths does not really make sense in genernal (i.e. I think that these statistics are only interesting for concrete values (instead of sub trees of a record (as it is the case for upper path analysis))).