Allow users to fill in NULL for initial water level in the schematisation

[leendertvanwolfswinkel]

At the moment, if you do not fill in v2_global_settings.initial_waterlevel, threedi-modelchecker will give an error.

This doesn't make sense; it is perfectly valid to run a simulation without any initial water levels. However, I don't know what happens if we remove this check from threedi-modelchecker. Will making the simulation template fail? Or will the simulation template be made and will the simulation crash?

So please test what happens after removing this check when generating a 3Di model with v2_global_settings.initial_waterlevel = NULL

[elisalle]

@leendertvanwolfswinkel this should probably be an issue in a different repo, since it involves testing on the api, not the modelchecker.