Hi Geir,
One approach would be to modify the XSLT stylesheet for your report so the template that prints EHT_Part Charactheristics applies another template to determine whether EHT_Part Charactheristics.eht_char_value is displayed.
For example, if this is the template in your stylesheet that displays EHT_Part Charactheristics...
... you can add a new template just for displaying the eht_char_val and add your specific criteria to the apply-templates tag.