Hi Hans,
There is a feature in RASTER > Raster Operations > Calculator > which can be used to reassign values to grid per specific conditions. I was looking at using the 'Replace with variable' expression syntax, and struggled a bit to get it to recognize using 'null' to replace all '0' values. It looks like it is not accepting null (string) as a variable.
Mitchell Holliday one of our subject matter experts kindly suggested to use the following syntax:
cond(Input_1 = 0, null, Input_1)
I've tested this and was able to assign null to all cells with '0' values successfully.
Hope this helps!
Dave