UO UI defaults fail schema validation

[vtnate]

• underground_cables_ratio is populated with null while the schema expects a number.
• max_number_of_lv_nodes_per_building is populated with null while the schema expects a number.

I didn't use those keys in the UI, they got added when I exported the geojson feature file with null value.

The UI shouldn't generate an invalid file.

Potential solutions:

1. Manually deleting those keys from the feature file makes everything happy again (this doesn't feel like a solution)
2. Update validation/schema to accept null for those keys
3. Update UI to not include those keys in the exported geojson file if they aren't being used
4. Update UI to populate with a no-op value (not sure this is possible)
5. Something else?

[vtnate]

@kflemin Do you have a preference between choices 1-5? Assuming 2 is the right answer, I made the attached PR. Please review at your convenience.

[kflemin]

@vtnate

I like option 3!