Non-AC buses do have load

[davide-f]

Checklist

• I am using the current main branch or the latest release. Please indicate.
• I am running on an up-to-date pypsa-earth environment. Update via conda env update -f envs/environment.yaml.

Describe the Bug

When running some countries with no capacity expansion, such as China, we discovered that non-AC nodes do have load, which is not expected.
This is likely because the flag "substation_lv" is not well threated.
That flag created in build_osm_network identifies buses with low-voltage that shall identify the substation with demand.
Then bus_regions may be created only for those substations and then the workflow should continue and verify consistency of results