Enable calculation of final 15.3.1 layer when no population data is available

[azvoleff]

From Pekka on Trends.Earth list:

"When trying to calculate the final combined assessment of 15.3.1, I cannot find or set any population data. It just says "No layers available in this region" for both broken down by sex and total population options. Executing this tool without population data is not allowed, nor is there any option to set this data yourself. I have also tested different regions - using custom shapefile, and pre-set country boundaries even for different countries - same result.

The tool should allow calculating the indicator without population data, as it is not needed for the SDG 15.3.1 reporting. So I am stuck here, while everything else is ready and solved."